As a fourteen-year-old we are introduced to Gilbert as a rather shy but kind individual. A bit of a cry-baby terrified by cats, he is completely and utterly devoted to his master, Oz Vessalius. Torn between the idea of being best friends with Oz (which he feels he doesn't deserve) and his servant, Gilbert is often lamenting or fretting over decisions and situations that clash the two in his mind. One example of this would be Oz's Coming of Age Ceremony. The manga starts out the afternoon before Oz's Coming of Age Ceremony with Oz and his younger sister, Ada, hiding from one of the servants responsible for readying the young master for the ceremony. Gilbert is found by himself, an unusual situation as he is almost always by Young Master Oz's side, and is questioned the whereabouts of Oz and Ada. Clearly trying to avoid the question, he is threatened with a cat and reveals the whereabouts of the two to be the Lake. However, once the servant leaves with the information, it's clear Gilbert has lied for Oz and Ada pop out of the dresser in the same room. Obviously uncomfortable with lying (and fear of being threatened with more cats), it's clear his devotion to Oz goes above and beyond that of any normal servant. Later, as the three of them are exploring the mansion grounds, Oz presents ceremonial clothes to Gilbert stating he wants him to be part of the ceremony. Gilbert feels he is unworthy, faced with doubts of self-worth and as a servant, not fit for such a role in the young master's Coming of Age Ceremony. However, Oz asks him not to do it as a servant but as his best friend. He leaves the decision up to Gilbert and Gilbert continues to fret, torn between wanting to please Oz and unable to overcome his own self-perception.

Despite his tendencies, however, his extreme devotion to Oz is clear and even causes him to have bouts of brave, if not foolhardy, actions. Although he later considers his actions a mistake, one instance of this is he wants to prove to Oz that his father doesn't hate him and thus confronts Master Zai Vessalius one snowy day while he's still at the Vessalius Household, begging him to see Oz before he leaves. Unfortunately, Gilbert's act of bravery and attempt to heal self-inflicting wounds Oz has created backfire, as it's only confirmed that Oz's fears of his father hating him are true.

"Do you think a servant can tell their master what to think?" - Zᴀɪ Vᴇssᴀʟɪᴜs, Rᴇᴛʀᴀᴄᴇ 10, Pᴀɢᴇ 36

"I'm sorry, but... you are not my master... Young Master Oz is!" - Gɪʟʙᴇʀᴛ, Rᴇᴛʀᴀᴄᴇ 10, Pᴀɢᴇ 36

To make matters worse, Oz is there to witness it in person and even tries to protect Gilbert when his father has Gilbert punished for his actions. Oz is also punished, not allowed to leave the household and is left alone, Gilbert being locked in the basement for three days to reflect on his actions. He's later rescued by Oscar Vessalius (Oz's uncle who is like a father to both of them) and comes to find Oz by himself reading, a wall already completely built around him from the experience three days prior.

"For three days I was locked up in the mansion's basement. I was rescued by Oscar. I went to Oz right away and what I saw was..." - Gɪʟʙᴇʀᴛ ᴛᴏ Aʟɪᴄᴇ (ɴᴀʀʀᴀᴛɪɴɢ ᴛʜᴇ ᴘᴀsᴛ ᴇᴠᴇɴᴛs ʜᴇ's ʀᴇᴛᴇʟʟɪɴɢ), Rᴇᴛʀᴀᴄᴇ 10, Pᴀɢᴇ 39

Gilbert breaks down in tears, claiming it to be his fault and doesn't understand why the Young Master is okay with what has happened.

"Hey, Gil. I've thought about it a lot. While you weren't here for those three days, the whole time I thought about myself. That's why it's fine. It's been like that from the beginning." - Oᴢ ᴛᴏ Gɪʟʙᴇʀᴛ ᴏɴ ʜɪs ʀᴇʟᴀᴛɪᴏɴsʜɪᴘ ᴡɪᴛʜ ʜɪs ғᴀᴛʜᴇʀ, Rᴇᴛʀᴀᴄᴇ 10, Pᴀɢᴇ 41

Gilbert's life as a child revolves around Oz and his personality and actions reflect this, breaking barriers in his character every so often for the young master. Even in his mold as a simple and kind child, the two of them treasure each other's presence and need little else in their lives until they're torn apart by Oz's banishment to the Abyss.

Ten years later, in the current setting, Gilbert has gone through quite a bit that has cracked and remodeled his personality from his time as a child. In order to reach the possibility of getting Oz back into his life, he's willing to get his hands dirty within the shadows of the Nightray Household and their duties within the Four Dukedoms. He acts cold and distant, even towards Oz when the boy initially wakes up after being returned from the Abyss. However, we see that most of it is a facade, especially with Oz. Once the truth of his identity is revealed to Oz and he admits to wanting to still serve the Vessalius heir despite now being part of the Nightray Household, his cold and distant attitude becomes extremely protective (maybe even overprotective). He can be harsh with the other but it's out of care and concern, worried about Oz's (unchanged) outlook on things (particularly of himself) and how oblivious he seems to be on what fate ultimately has in store for him.

At times, Gilbert can still be seen as his fourteen-year-old self, crying easily when drunk and calling Oz "Young Master" (for the most part, despite being his servant, he simply calls Oz "Oz" when addressing him, although refers to him as "my master" from time to time when discussing him with others, such as Alice). For Oz, it's an extra comfort to know that deep down Gilbert is still the same person he knew before being dropped into the Abyss. Despite ten years of trials and hardships, Gilbert has still managed to retain a part of himself that even he himself thought that he had lost as being familiar to Oz. Yet, Gilbert is still caring and kind, often acting as an older brother of sorts to both Oz and even Alice, the latter with whom he has an odd relationship with. Oz thinks that Gilbert has a soft spot for her, but it's currently unknown if it's true. It's known that he has a strong desire to kill her at one point in order to save Oz from the fate of being an illegal contractor.

Gilbert holds a great amount of respect for his younger brother, Vincent Nightray, particularly because of his brother being extremely talented. Vincent's ability to surpass Gilbert's skills with a revolver that had taken him years in a matter of months is of particular highlight. It's also clear he cares for his younger brother, although to what level is unknown, particularly with recent chapters having them on opposing sides. However, when the two were much much younger (before Gilbert ever met Oz), it's shown that while Gilbert knew his life would be easier without Vincent if he had just left him alone to fend for himself, Gilbert was never able to do so.

Now older, he also harbors an intense hatred for Zai Vessalius, Oz's father. It extends past the point of hatred, really, and also crosses into blatant disrespect for the Vessalius Household leader. He goes as far to shoot at the other when he makes an appearance before Oz, warning Zai to stay away from the boy. If not for Xerxes Break's intervention, the confrontation would have erupted into a battle between their chains (Zai's being Gryphon and Gilbert's being Raven). Zai well aware of Gilbert's desire to rid Oz of the person who has caused him the most pain. For Gilbert, it's also to rid himself of the man who is responsible for him losing Oz in the first place.

In recent chapters, Gilbert went through a massive character growth from what he was. During his attempt to save Oz even after learning the truth about Jack Vessalius and about Glen Baskerville being his master, Glen commands him to shoot Oz. Gilbert's hand moves on its own before he can even blink and watches himself shoot the absolute single most important person to him. He's left in a daze for a bit before finally having to face what he did indeed do. Now that he remembers everything from his past, he makes his decision to have Raven judge him for is sin (shooting Oz) and in the end has Raven burn his left arm off. Gilbert also destroys his initial contract with Raven and forges a new direct one.

When we finally see Gilbert after he is engulfed in Raven's flames, it's him saving Oz from execution at the hands of Glen Baskerville. It's here that everything about Gilbert becomes much more clear: his desire to stay and protect Oz has not changed and never will, but when he could not remember his past it led to self-doubt and a fear that something horrible inside of him would hurt Oz. Despite his great care of and respect for Glen, he tells Glen that he cannot serve him any longer and has no intention of giving up what he's gained. This is also the first time he really calls Oz "Oz" rather than having the hesitance of needing to call him "master". Gilbert finally can articulate his feelings for Oz (well, a little more than before, anyway) and he doesn't just hold loyalty to Oz as a master, Oz is his best friend and most precious regardless of "rank" and who or what he is.

With Gilbert's transformation, it seems a few things have adjusted in his personality and body language (but there is very little to be seen past this revelation as of right now, too). He is sharper, noticing things that he would normally not (or would convince himself to overlook or even notice the subtlest changes in Oz such as when Jack possesses him again without warning). His entire world is even more centered around Oz, now, as well (but he cares a lot for Alice, too, and actually hugs her along with Oz when she returns). He's a man that's chosen a path he is confident in and is ready to do what it takes without hesitation to get himself there (and protect Oz because without Oz he really doesn't have much nor would he want to).

Oz often teases Gilbert still, and Gilbert is easily riled up as a result. He also has a history of engaging in bickering (and sometimes physical) arguments with Alice, the two often disagreeing and fighting over their opinions while insulting each other. Oz never takes sides when it happens, often appearing oblivious, which just fuels the fire between the two of them (and more often than not it's for Oz's attention they fight).

Gilbert, even as an adult, never gets over his fear of cats. This is revealed when Oz and Gilbert enter the realm of Chesire to go after Alice and Break, who have already been "captured" (or more like Break used Alice as bait to lure out Chesire for his own intentions).

"Gil, what now? ...Huh?!"
"Cat... I didn't know it was a cat..."
"...Don't tell me that your cat phobia hasn't been cured? ...HEY, WAKE UP." - Oᴢ ᴀɴᴅ Gɪʟʙᴇʀᴛ, Rᴇᴛʀᴀᴄᴇ 16, Pᴀɢᴇ 10

As part of the Nightray family, Gilbert actually does hold some affection towards his youngest brother, Elliot Nightray, who is a pureblood Nightray, unlike he and Vincent. Vincent mentions in later chapters after a certain event happens that he thanks Elliot for being there otherwise Gilbert would never have ever been able to smile in the Nightray Household.

Random facts include he is an amazing cook and, according to Xerxes Break, that he took up smoking because he idolizes Oscar Vessalius. Rufus Barma mentions that although he attempted to quit smoking eight times, he failed. The hat he always wears with his default outfit was given to him by Ada Vessalius and is very precious to him. It's important enough that when Oz loses it in the city, Gilbert drags both Oz and Alice to search for it. When it's found to be in the hands of some street thugs, Oz decides on them all having an arm-wrestling match for who has rightful possession of the hat.