Salti is Phineas' mermaid mother. She's a very kind and girly-girl type who loves to fuss over little kids.
She knew Kavonn growing up, which happened rather fast from Kavonn's perspective since mermaids of her type grow into adulthood much faster than humans. Kavonn had feelings for her but she did not quite return them due to outgrowing him so quickly. Much to his consternation, he will always be that goofy childhood friend who came back only to now be her son's goofy childhood friend.
Salti's husband is a huge, hulking monster, but he's a really nice guy once you get past his terrible smile.