Altair is a good soldier but only because she's black. If you have Jerome i'd use her + Jerome but otherwise Katie/Aria/Lieselotte.

The male event soldier is not that great. He has good stats, sure, but his skill is the same as Altair which brings a problem. You have to wait for him to generate all his UP before his skill starts cooling, meaning his UP generation is abyssmal compared to a soldier with instant +15 UP.

Altair is rescued from this thanks to being black so her skill is available immediately.