SerTapTap
Member
Basically just look in curse rooms and try and get daels with the devil as often as you can and you'll be guppy in no time
Also open all red chests. I found someone who apparently ignores red chests. Yes, sometimes there are spiders, but more often than not they're worth it. They can also have soul hearts, guppy items and other goodies. I've yet to take damage from opening one.
The Left Hand can help with Guppy, it turns unlocked chests into red chests.
In all my hours of playing the original I don't think I ever became Guppy. Think I was wary of picking up the dead cat. I should give it a go sometime.
Dead Cat counts, somewhat unfortunately. Can't blame you, it can end runs. Even as Eve it's a risky item since it's basically OHKO in the womb and onwards.