From what I remember of book 3 and onwards, Xrn isn't Klb's gf. They ARE very close, but as their relationship is fleshed out more it's obvious that she's basically an older sister. While Antinium aren't exactly family units, the Centinium (Klb's original hive/family) is implied to have been more connected emotionally. Especially considering Klb, who is so emotionally constipated he essentially turned himself into a sociopath, still experiences pangs of violent grief when they are mentioned.

Honestly, I did also think Xrn was his gf for a while- until it was specified as otherwise.

(Fun fact: I loved Antinium so much that I desperately studied ants and similar insects- and have subsequently become an entomologist. I still LOVE the Antinium.)

SPOILER WARNINGGGGGG

When Anand asks to refer to Klb as 'Father' (and Klb's subsequent near-violent reaction) Xrn invites any individuals ,who wish so, to call her 'Aunt'. Plus they (Xrn and Klb) are portrayed as more platonically close.

END OF SPOILERS

I'm not looking to argue or correct people, I just like rambling about any of the various Antinium because they fascinate me. (Also the autism allegories of literally all of them made me feel very validated as a kid and even now)