I Hope this will help a little about Bpet.
So Far that i've known about Bpet
1. Bpet did add status and some bpet status excels the other bpet. for Example
Fire Bpet having more stats on Attack Speed and Max HP
Dark Bpet Having More stats on Damage and Max AP
Light Bpet Having more stats on Magic and Movement Speed
Water Bpet Having more stats on Evasion and Defence (havent got one, was askin from my Friend)
2. Formula ? idk about the Formula..
as far as i know about the addition status for bpet
it goes up 1 or 2 for damage (dark Bpet) for every level, and some magic point for every 1 or 2 level on Light Spirit
Fire Spirit gain 1 attk speed every 5 levels. Light Spirit Gain 1 Move Speed every 5 or 7 level (Cant Remember)
and I Think the Grade of the Bpet hav some effect from the status addition (higher Grade get more stat than lower grade one)
3. Lv.70 Dark Spirit with Grade A = having dmg status addition about 138 (if im not Mistaken)
Lv.45 Dark Spirit with Grade AA = Having dmg status addition about 130
the same level with Differ grade should be different
4. I think bpet hav the same amount of exp needed for each level no matter what grade it is, the differences was on the Bpet status
Bpet with Grade A status will hav lower status (Dmg and def) than Grade AA one,
with that fact above. ofcourse the time for killing monster / grinding place for differ Grade Bpet will be different..
so.. higher grade bpet should be faster than Lower Grade Bpet.
5. look like they starting to appeared at Grabbit / Golden chest... idk about monster drop (normal Gear), havent got one from monster.
Thanks for your time reading.
Lucy