Here's the lore I've been thinking about and the reason why our Overlord targetted them specifically. I was thinking our boy is from a Earth-like planet that was ravaged so hard by the eaters. Feel free to expand into their respective guardian stories.
Nova Serena - Let the eaters 'live in peace' causing them to wreck havoc in space stations, colonies and nearby planets. Her reason is that these are just animals that have the same right to live as sentient beings.
Pyra Valeria - Extorts colonies and stations for protection money (resources) in exchange for dealing with the eaters. Abandons poor colonies that can't pay with tech or resources.
Glacia Yuki - 'Guides' eaters with black ops soldiers (so that she has plausible deniability in case they are caught) into other planets then feign ignorance when those planets suffer damage and loss of life.
Kessa Sundari - Hunts moon-eaters and claims she is a powerful hunter (even if a well equipped squad can deal with these). She's actually scared of the bigger ones like planet-eaters and sun-eaters but hunts the lesser eaters so that others won't call her out.
Nyx Isolde - Too nihilistic to deal with the eaters as she saw both rebels and councils unleash the eaters on each other throughout the centuries.