Kelvin is a disruptor/brawler. His job is to get into the fight, stun/slow everything, then start pounding away and comping low health enemies to kill them. He’s also a bit of a controller with his Ice Wall: he can block reinforcements, prevent enemies from running, or stun/damage them depending on how he places it.
Alani is an attacker and burst healer with CC abilities. Her job is to push the front line, heal when she’s able to, and prevent the enemy team from counter-attacking, much like OM. Like OM, she’s not a brawler and should never engage in 1v1 unless she knows she has the upper hand (low health enemy). But she’s great in 2v1 and 3v2 because she can stun/slow the enemy while her teammate finishes them off.
Ernest is mobile artillery and will always be such no matter which mode his egg is in. He’s like Benedict, except he can buff his teammates.
Reyna is true support: her job is to keep her teammates alive while making it easier for them to kill the enemy through her priority target. A Reyna without a teammate nearby is generally a dead one.