Well I'd go for this, if you're a mage I recommend either Victoria or Frederico or Kanako to be your support (tho I prefer Angela more, but she isn't rb'able).

If you're a fighter, I recommend Niss. Niss is good, but she isn't burstable and you'll need to scroll her for her to be good.

I myself have pets like this on my chars:
Fire wit = Vic, Fred and Angela.
Fire killers = Niss and Shasha.
Water priest = Xaolan and Qlaya.
Earth knight mage = Elin and Angela.

Reason why 2 of my chars got Angela is 'cus I don't alwaya have 2 of them in team or my water isn't along, so if my wit needs to use a fighter pet, my earth can do the double support job. :)

But this is totally my own opinion, though I planned/thought it all through, not to regret anything later. So far, so good, couldn't be any better. :)

Good luck on deciding! I am sure you'll find your method and preferrable pet by the time. :)

Edit:
PS: The reason why I have chosen pets like these is cus of the good combination, I can use and switch them depending on the situation, always helps me out. If there are many sealers in battle, I use sealer pets etc. I planned them to be helpful for every situation possible.

My support pets are tanks and fighter pets mainly pure damage. :)