Author should use this to give him a girl 'friend', that can develop his views on romance and relationships even if the author doesn't want him get a girlfriend. Gonna assume that we not gonna see this character ever again unless the author decides to continue this pass the gag.
I think usato just wanted to trust nea before she revealed her true identity. I mean usato usually trust someone unless they do something suspicious. I don't think getting attack by zombies is quite suspicious enough for usato to assume it was nea.