Keeping in mind the usual disclaimers for advice…
2nd Proteus could indeed be useful in challenge events, trials, raids (depending on your arena) and 4* tournaments. I’ve done it myself before (having 2 Proteus completely block an enemy defense team, or having them take turns to make sure triple bosses never fire). And of course, great for Wars, so you can put Proteus on 2 attack teams.
BUT!
I would go variety first. You have two great “utility” Purples (Proteus as mana blocker, Rigard as healer/cleanser). Gafar adds some good direct damage to your Purple bench.
For a longest time my only Purples were Proteus and Sabina, and I struggled when stacking Purple in Raids because of the lack of direct damage special skills. I ended up stacking Purple as the 2 in 3/2, but never as the 3, until I pulled Seshat.
so I say, Gafar over 2nd Proteus.
The Cat is interesting, but can wait for when you have more variety.