"Well--!" Madoka starts, suddenly looking much more decisive. Her expression even shifts to match the tone she's using. Look at this determined young little teenager.

"You won't ever have to be alone here!" Mostly since Prompto probably has plenty of friends here already, she thinks. Or just because people on the train are really nice, and she doubts anyone would hesitate to befriend him.

But if he needs a bit more reassurance than that, then..

"Because I'm right here with you, aren't I?" And sure, Madoka could disappear at any time. Any of them could. But Madoka has been stuck on the train for so long now that she's willing to bet on these odds.