Outwardly, Sei is Kiravai, albeit oddly coloured - inwardly, he is in fact a member of a "species" (after a fashion) that the Kiravai call Kirasiinu, which in human terms equates to "android". Fully sentient and self aware, he is a somewhat troubled individual - concerned that he is unwittingly moulding Mirii into the likeness of the sweetheart he was forced to leave behind in Coalition space. Their holiday was his idea - to try and encourage Mirii to become more autonomous, more self-directed. He wasn't expecting her to suddenly be forced to rely entirely on her own intuition quite so dramatically...
Mirii is a young Kirasiinu pen struggling with her own autonomy. Even in Siinu terms, she is young; her adoptive parents helped her begin the journey of self-discovery, but at this point in time she is still very... well... "simple".
Officially, Suni is Mirii's pet; unofficially, the little umasiinu is more of a guide and guardian, acting to keep her safe and offer support when Sei is not present. Similar to its friends, it is a synthetic construct, and not only sentient but quite strongly intelligent, but as a furred quadruped, most assume it to simply be a variant gii-le, thus "just an animal". It has not decided on any "gender identity", partly due to its lack of any form of genitalia, and happily identifies as an "it", although its behaviour patterns mean that it is also referred to in the masculine (it doesn't mind being called "he"). It does have the capacity to speak, and yet understands that Kiravai get upset with animals speaking in Ve-hei'ya, and thus remains primarily mute out of respect; when it "speaks", it is only in a variety of chirps and barks.