Adorkable: Face it - he's a mecha anime addict, loves to eat, a Butt Monkey, rather emotional, often unintentionally funny, yet still remains a big Badass. He can't be anything but this.
Badass: Well... It's a World of Badass after all.
Crouching Moron, Hidden Badass: Ye, he IS quite a dope and can be really slow sometimes, which hinders the Club's missions sometimes, but he makes up for it with his quick logic and can stand the ground on Jellal when he needs to.
Took A Level In Badass: Going from a Support Party Member in Fairytales to a huge Combat Support in Next Generation is quite a development for a Flat Character.
Berserk Button: Do NOT try to harm Amethyst in ANY. WAY. EVER. Unless you really are stupid or even wanting a death wish along the way. As the Tartarus Magicians learns this...
Boke and Tsukkomi Routine: Diamond's the boke.
Straight Man and Wise Guy: He's the Wise Guy to Pearl's Straight Man.
Brilliant, but Lazy: He's actually very intuitive for his age, but rarely shows it. Eventually drops the lazy part by the time Next Generation rolls in.
Can't Catch Up
Character Development: Although not quite shown in Fairytales but in Next Generation shows that Diamond's finally losing his Big Eater part (although he still eats a lot, but not always) and is shown to be a hell lot a Good Parent towards Amethyst than the others..
Character Tics: Before Next Generation, he slaps his cheeks when he feels determined.
Cheerful Child: Made more apparent with the fact Diamond's the only person out of the Theater Club who doesn't have any troubled past or going through an emotional trauma the others got.
Chronic Hero Syndrome
Cool Sword: His Narukami.
Combat Medic/Combat Support
Healing Hands: Some of his Light spells are able to do this.
Light 'Em Up: His main magic.
Holy Hand Grenade
Mighty Glacier: He's this for a Light Magician and his Pokémon are all this. However...
Lightning Bruiser: When Diamond's really pissed, namely when Amethyst is hurt, he's definitely this.
Summon Magic: Able to summon Kirin and Alexander, two of the Summon Beasts of Light.
Support Party Member: Acts as one whenever the Club is on a mission.
Dork Knight: Despite not being a real knight, he qualifies as this.
Flat Character: In Fairytales, kinda. Given unlike the others, Diamond's rarely shown to have a new development by the end of the series although Next Generation lessens this due to already grown up.
Friend to All Living Things
Get A Hold Of Yourself, Man!: To Pearl in Third Year during the latter's Heroic BSOD moment and every member of the Theater Club in Next Generation when their children are risking their life.
Generation Xerox: His daughter, Amethyst inherited a few notable traits from him, namely his huge appetite and being a Supreme Chef.
Good Parents: While most people think Diamond would be a Bumbling Dad towards Amethyst, he's actually this. And is pretty much the only Good Parent out of all Fairytales characters.
He Is All Grown Up: In Next Generation like most of the casts.
Hero-Worshipper: Admires Red/Shouma as a child and later to Satoshi, when the latter saves him.
Heterosexual Life Partners: With Pearl.
Bash Brothers: This too.
Inelegant Blubbering: Whenever he's crying, it tends to be this.
Innocent Prodigy: He is much deeper and understanding than he appears.
Iron Buttmonkey: As the boke of the manzai routine, he has to be.
Like A Daughter To Me: Doesn't mind of Sandalphon/Himeka's existence and greatly considers her like a daughter to him just as much as Amethyst liken her to a big sister.
Older And Wiser: In Next Generation.
Papa Wolf: Diamond's reaction when a Tartarus Magician hurts Amethyst in front of him? He instantly summoned Alexander, one of the most powerful Summon Beast. Cue the Magicians' Oh Crap reaction when they saw this.
Team Chef: A good one too. He can actually made a normal food taste like a high-quality one (from Gold anyways), not to mention a strawberry pie for Satoshi when he got depressed. This is eventually passed down to his daughter, Amethyst.