[Autumn]

Summoner's Affinity skills are increasing a Youkai's stats even while no summoner class is equipped, most prominently available to Guardian Spirits.

To replicate:
Look at a Guardian Spirit's HP
Buy the +5 stat passive in Summoner for that youkai type
Heal up at  an inn
Guardian Spirit's HP increases, this also applies in battle with the +5 all stats.

[AkaInuHime]

rip youkai hit even more

[Neus]

This has been corrected in 3.01f.