Serra the Benevolent

In a Two-Headed Giant game, damage dealt to you and/or your teammate that would reduce your team's life total to less than 1 reduces it to 1 instead, as long as you control a creature. It doesn't matter whether your teammate controls a creature, even if they're the player being dealt damage

Serra's first ability affects only creatures you control with flying at the time it resolves. Creatures you begin to control later in the turn or that gain flying later in the turn won't get +1/+1.