Pull for Robin?

Don't pull for Grima unless you like Grima for being Grima. I'm pulling for him because I like him, not because he's a dragon, or an armored unit, or has good skills.

Unless you really like him, want more bulk for your dragons or his skills for fodder, I'd say no.

if you like him, pull. If not, hold.

I'd pull just for vengeful fighter

depends, what do you need? if you're pulling him because you want a green drag, its not worth. Myrrh is a beast already. if you need a green armor, Hector is already pretty decent but as a dude who likes mixed armors, grima definitely outshines him there so yeah.

I desperately needed a green armor so I could do the armor quests for GHB so he's serving me well.

I thought the same thing and then when I tried pulling for him I got an extra Myrrh with perfect IVs. It then hit me that I already had what I needed. Although his kit is alluring and he is a fairly good unit, I didn't really need him. I already have many good green units, particularly ones I'd use as tanks. So I decided to opt out and save my orbs. Sounds to me like you also already have good units that would fill a similar role. If you want him for collecting purposes or simply because you like Grima as a character, then go for it. But imo I don't think that you really have to have him, since you already got the units that would serve his role.