Who can forget her after playing FFVII? Like her or hate her, you probably won't. Aeris Gainsborough, a mere flower seller, joins Avalanche to save the Planet. The last surviving Cetra, only she alone can stop Sephiroth from destroying the Planet. A caring and loving person, she has an attitude that can always lift those around her. In the end, she sacrificed herself to Sephiroth's blade, in order to help save the Planet.
Why no.5?  She'd do anything for anyone but herself. She gave herself up to save Marlene. She risked herself going into Don Corneo's mansion to help find Tifa. She sacrificed herself to save the Planet. People like Aeris are very rare. Would you do the same as she did?