He only appears in-game after the player claims his Character Unlock Mission reward for defeating Standard Duelists 100 times. In the beta, he instead only appeared after the player reached Stage 20.
Regional champion Weevil Underwood loves to buzz to victory with his InsectDeck. Nothing will stop Weevil from getting a win, even if it means throwing Yugi's cards into the ocean or messing with Joey's Deck. One of his favorite techniques is an Insect Combo using "Great Moth" and "Insect Queen".
Immediately after the player claims the reward for Weevil Underwood's appropriate Character Unlock Mission, they will encounter Weevil for the first time, who is engaged in a Duel with Mickey. After Weevil's apparent victory, he tells the player that they've just witnessed the power of his Insect Deck, which he'll use in Duel World to become a champion once again.
Until the player manages to defeat Weevil at least once, whenever they challenge Weevil to a Duel, he will declare that he is growing accustomed to Duel World, with it being infested with "weak little larvae." He considers the player as the weakest of them all, and predicts that becoming the champion of Duel World will be easy. He then concludes his dialogue with snickering.
Until the player manages to defeat Weevil at least once, whenever they lose to him, the latter briefly sniggers before declaring that their Duel was too easy - the player never stood a chance against his bugs. He further brags that insects are invincible since they've been around for millions of years, and that no card can defeat them. He then taunts the player, saying that if he were the player, he'd give up and "buzz back home," before sniggering again.
Upon Weevil's first defeat, he gives a brief yell of shock before stammering that he can't believe his insects lost. He attempts to defend himself, saying that his defeat doesn't prove anything, and especially doesn't mean he's a weakling. He then has a brief surge of confidence, as he vows that he will be champion, but he ends up concluding his dialogue with a sniffle.
After the player redeems their final unclaimed reward for Weevil Underwood's Character Unlock Missions, they will encounter Weevil again, who chuckles before informing the player that he has been "spinning" a winning strategy for himself to become the champion, and reveals the "Insect Queen" card to the player as his perfect plan. He goes on to declare that he will serve his Queen by literally doing whatever it takes to win, to the extent that he'll make sure his victory is guaranteed before the first card is drawn, concluding his dialogue with a titter.
Whenever Weevil Summons "Gokibore", a cut-in frame of his face will briefly appear, and he'll announce "Check out this rarest of rare cards!" followed by "Gokibore!"
Him referring to "Gokibore" as a rare card is likely a reference to how he promised his accomplice a rare card as payment for his services but instead gave him "Gokibore".
Whenever Weevil Underwood Summons "Insect Queen", a cut-in frame of Weevil will briefly appear, and he'll announce "Heeheehee... Here comes the ruler of the entire insect kingdom!" followed by "I Summon Insect Queen!" If that player hasn't already Summoned "Insect Queen" during that Duel, a cutscene of "Insect Queen" being Summoned will play afterwards.
Most of the time when he declares an attack with that monster, he announces "Her Highness attacks!" followed by "Queen's Toxic Breath!"
Whenever he activates that monster's effect, he announces "Insect Queen's effect activates!" followed by "When Her Majesty destroys your monster in battle, she lays an Insect Monster Token!"
Whenever Weevil Underwood Summons "Perfectly Ultimate Great Moth", a cut-in frame of Weevil will briefly appear, and he'll announce "This is nothing that you've ever faced before or will ever face again!" followed by "Perfectly Ultimate Great Moth!"
Most of the time when he declares an attack with that monster, he announces "Perfectly Ultimate Great Moth! Attack!" followed by "Perfect Moth Hurricane!"
Whenever Weevil activates either effect of "Kiseitai", he announces "Kiseitai's effect activates!" followed by "It attaches on to monsters who attack me to feed me Life Points!" [sic]
Whenever Weevil activates the Flip effect of "Parasite Paracide", he announces "Parasite Paracide's effect activates!" followed by "It infects monsters you Summon, transforming them into Insect-Types!"
Whenever Weevil activates the effect of "Skull-Mark Ladybug", he announces "Skull-Mark Ladybug's effect activates!" followed by "When it's placed in the Graveyard, I regain 1000 Life Points."
Whenever Weevil activates the effect of "Cocoon of Evolution" by equipping it to "Petit Moth", he announces "Here's my ace up the sleeve!" followed by "Cocoon of Evolution's effect activates!"
Whenever Weevil activates "Eradicating Aerosol", he announces "The Spell Card, Eradicating Aerosol!" followed by "It destroys Insect-Type monsters!"
Whenever Weevil activates "Infinite Dismissal" (or its effect), he announces "I activate my Trap Card, Infinite Dismissal!"