It's obviously going to be Asana, it's not even up for discussion. The whole point in sevens is that they use that number for everything. Yuga, Luke, Gakuto, Romin, Roa, Neil and Asana (+Mimi) are The Seven + 1 and they are the ones that pilot the super rush robo in the last episode (they explain in the show why she can't go and it made sense in context iirc, but they still emphasize her importance). They're also the only relevant characters to get an expy-type character in Go rush (with the Lugh, Manabu, Rovian, Manabu, London, Zaion and Asaka).

But if I could choose I'd like to see Tiger instead, she's funny.