Normally, Mustard wears a green shirt with a blue paw print and yellow undershirt as well as white pants. Like Leon, he wears sneakers and also wears a bowler hat with green highlights.
When he gets serious, he takes off his jacket and hat, and his eyebrows which normally flap downwards now point upwards. His hair is wrapped in a ponytail with a red headband and he will strike a kung fu pose.
When he was young, Mustard used to have a hot pursue on accomplishing for the best, and therefore had an overly strict standard on himself and others. But after his retirement, he has already loosened himself up, though he still gets ecstatic whenever he gets the chance to battle a worthy opponent. He prefers playing fair and square and dislikes people who rig and cheat.
Although mostly a hot-blooded battler on the battlefield, he also has quite an addiction to playing games, as seen that he is always playing Pokémon Quest with Joy-Cons in his room.
Prior to the Isle of Armor
Prior to Dynamax being introduced into Gym Battles, Mustard had reigned the title of champion for 18 years, and nobody, even his student Leon, can beat his record. He is also the first to discover Leon's talent. 50 years ago, he was once a Fighting-type Gym Leader who earned and reigned the title of Champion and had triumphed against his rival who was then the Fairy-type Gym Leader. However, after the death of his partner Pokémon, he lost his shine and his winning streak. He didn't retire until the then Chairman gave him a request of faking a match to win, which made him quit entirely. He later purchased and named the Isle of Armor, where he opened his dojo with his wife Honey.
Isle of Armor
After the player clears all 3 trials, he will give the player a Kubfu and permission to have a buddy on the isle of armour. The player must raise Kubfu's Friendship, then fight their way through one of Tower of Two Fists to evolve Kubfu into Urshifu (with him and his Kubfu with a Focus Sash being the final opponent). Once the player's Urshifu gains the ability to Gigantamax, he will challenge the player in one more battle where he goes all out.
- His name derives from the plant and condiment mustard. The word "mustard" also hints towards "master", hence his class as Dojo Master.
- Mustard and Leon are the only characters whose uniform numbers are not a wordplay in Japanese and only consist of a single digit.
- Mustard's uniform number is "0", as seen in his rare League Card. This may be a reference to him being the Champion before Leon, whose uniform number is "1".
- He is the only Champion, be it former or current, to use a legendary Pokémon.
- The fact that he goes all out is similar to Goku from Dragon Ball, as they change form when holding nothing back.
- The fact that his eyebrows change is similar to Dante Kouryu from Beyblade Burst Rise, whose blue hair highlight emits blue flame like effects when he gets serious, as well as every Burst protagonist since Aiger Akabane, whose tied up hair breaks the band and changes from brown to crimson when he goes all out.
- He usually refers himself as "little old me".