I've started playing an Unchained Summoner with my play group, and I'm trying to round out the last little bits of by character. When I was looking through traits, I saw Faith trait Loyalty across Lifetimes, which reads:
You and your eidolon share a link that seems to span multiple lifetimes. Your eidolon treats its Constitution score as if it were 2 points higher for the purposes of determining when its negative hit point total sends it back to its home plane. In addition, your eidolon gains a +1 trait bonus on Will saves against enchantment spells and effects.
I don't think this means that the Eidolon gains a permanent +2 to Con, but I'm not sure how to interpret this trait. What does it actually do?