The first time Bakugo uses his quirk on Izuku, Lee swiftly restrains the bomb-boy and calls the four-and-a-half year old out on getting started on their villain origin story early. Because bullying is never the act of a hero, and setting an explosion off on Izuku’s shoulder (and leaving a nasty burn) is bullying at the very least.
Bakugo denies villainous tendencies at all, that he’s going to be the next number one hero, King Explosion Murder (which Lee points out sounds like a villain name to him…) and stomps off in a huff once he’s released from Lee’s hold. Bakugo doesn’t try to use his quirk on Lee, because Lee is bigger, older, and faster, even if he’s otherwise as much of a quirkless loser as Deku.
Izuku denies Bakugo’s villainous behaviour, even though not ten seconds before he’d been telling Bakugo that what he’d been doing was wrong, and standing between Bakugo and another kid who’d been crying because of Bakugo using his quirk on him.
Lee ignores these protestations, escorts both kids to the school nurse, reports Bakugo’s behaviour to her, and then walks his little cousin home to the apartment building they both live in - where he dobs in Bakugo’s bad behaviour to Inko as well.
Inko is not impressed with the son of her best friend, and passes this on to Mitsuki herself next time they catch up. In the mean time, Lee shows Izuku what he’s already doing to get ready to become a hero when he grows up, and offers for Izuku to join him in that preparation.
Izuku, being a smart little bean, jumps on the offer, and counters with his very first Hero Analysis for the Future notebook. It’s written in crayon, but Lee can see that it’s already an indication of just how smart his little cousin is and how good he’s going to be at this after a few years of keeping up this kind of habit, so agrees to trade physical training for brain training, because good heroes have to be smart as well as strong!
They’re gonna call their agency Green Beans. Lee will be The Hard Work Hero: Ganbaru! (頑張る, lit. ‘stand firm’). Izuku will be The Can-Do Hero: Ganbare! (がんばれ, Hang in there!).
Lee, being a reincarnated Konoha ninja, also makes sure that Izuku looks up to more heroes than All Might. Sure, taking after Gai-Senei had worked out for Lee, but All Might had… well, Lee knew a fake smile when he saw one, and what a person bottling up trauma looked like. Gai-Sensei had been full of life and passion and the Power Of Youth and a great teacher as well. All Might… For all his smiles, Lee thought the man looked like he was dying inside and doing his best to hide it. That wasn’t Youthful. It wasn’t even Hip! A hero for his adorable cousin to look up to and try and emulate had to be at least Hip, though Youthful was the preference, naturally.
(When Lee gets to UA, he decides that Aizawa-Sensei is definitely Hip, and maybe Vlad-Sensei isn’t as Youthful as Lee would like, but he’s still glad to have once again gotten the Youthful sensei rather than the Hip one. Izuku doesn’t care if he gets a Hip or Youthful sensei, he just wants to get into UA’s hero course with his super Cool and Youthful cousin!)