If you are a free to play player, then don’t go for Woofie, he is not worth the time it takes to gather this many gems. If you are pay to win, and spend a decent amount of money on gems, then you should save up/buy 13,800 gems and buy 25 Woofie crates, which should get you an average of 2,275 openings with Woofie shards. The average amount of shards from a summon gate is 2.75/gate, meaning that with average luck you should get about 6 shards in the 25 openings, and with the 5 guaranteed shards you should unlock Woofie. Woofie is in the restricted section of pets, meaning that once you unlock him he becomes available in regular summon gates.
If you want a pet that will actually help you in your runs, then Boogie is the best, he is almost completely worthless if he is not 4 star, but once you get him to 4 stars he easily becomes the best, void shot will work like a mini suppressive fire, getting rid of all bullets that his shots hit. With Lizzie at level 50 all your pets will attack 8% faster, allowing Boogie to delete more bullets. Since Boogie is only good at 4 stars and it is quite hard to get that many shards for free to play players, I would avoid actively going for Boogie as a free to play player, but just gather his shards in the background, if you do get him to 4 stars then he is definitely the best though
Another great option is Celeste, she will heal you by roughly 3.9% on every 10th shot, and at 3 stars she gets the perk double shot, essentially only needing 5 shots to heal. At 4 stars Celeste gets combat medic, increasing her healing from ~3.9% to ~4.5%. With the added attack speed boosts given by 2 and 3 star you can get a great constant source of health with her. Celeste is the only fully premium pet, so if you want to unlock her, much less star her up, that’s going to be really expensive. If you want to spend a lot of money on this game then she’s a great option, but she’s going to cost a lot, to unlock her, with the worst luck it will cost 163.800 gems, this is if you get 0 shard drops in all openings, and go for the 5 guaranteed shards 12 times.
All the other pets aren’t too important, as all the above are much better. If you have trouble with gems, or don’t have Boogie at 4 stars then Fern is a good alternative, when you unlock her at at 1 star you can get stun shots, which has a roughly 95% chance of stunning enemies, and a roughly 33.3% chance of stunning a boss for exactly 1 second, which can be a great help with strong enemies, and it’s a nice bonus for bosses. You can also use Pickles, his ice shot is very nice in slowing down the melee enemies, and it unlocks at 1 star, so it shouldn’t take long to get. If you’re doing endless bosses Bean is one of the best ones, her mix of focus shots and multishot and double barrel make her quite good against rooms with single or few enemies. These are the best options for free to play players, as they really don’t take long to get