At a glance I could make sense of Rion not having a summoning contract since she's human, but... Sera does have a contract? I mean yes she's a demon, but... what?
Wait... Isn't [IMOUTO] a summoned character? Why doesn't MC have a summoner contract or any connection to her whatsoever? Odd, considering the fact that he has this link with pretty much everyone in the main cast.
@Reimos It's probably just down to the differences in the nature of the spell; one is meant to work with monsters, who usually wouldn't have a way of communicating with their summoner normally, so it establishes a telepathic link, whereas the other is explicitly meant to work with humans, so it probably slaps them with a universal translator in the summoning process and that's enough to communicate with the summoner.
Different kind of summoning. One is just pulling someone over from another world while one is actually subjugating and bending the creature to your will.