The pre-retaliate (pretaliate?) is very strong. If it defeats the attacker, Billy doesn't take any damage. Three allies, a hero, and Web of Life and Destiny lets him hit for 5, which is a massive amount for effectively no consequential damage. Even if he's just doing an easy 3 damage with no consequential, that's still very good. Against big minions (e.g. Tiger Shark), you can chip them down a little with other attacks and let Billy clean them up.
In a pinch, 5 HP is enough to have a plausible chance of surviving a hit from some villains, and he still gets to pre-retaliate even if he goes down.
Bonus: in addition to all that, he can also slowly remove 4 threat by thwarting. Which is an area Protection often struggles with.
One possible anti-synergy is that Billy's main value is in killing minions: some scenarios are light on minions, and some heroes (Valkyrie) and decks (No Quarter) really want to be doing the minion killing. So if you're on a team with one of those players, you may end up settling for just thwarting and chump blocking, which is still solid.
Hidden weakness: Billy doesn't defeat minions until they're actually attacking. This can be awkward for minions that hinder you with Guard or Patrol, or if you want to flip down to Alter Ego (which will cause the minions to scheme instead of attack).
Overall: A+, would UK again.