It's funny I have given this thought. Runea would be a 4 star who isn't super great to use but when you promote her to 5 stars her art gets super nice. She'd be closer to a glass cannon unit. She's def a unit you use cause you like her, not because she's great in the flow of battle. She would get one alt down the line as a 5 star that requires you to complete an event for her to show you care. Roo is a 5 star April Fools unit who has low attack and defense but weirdly very fast for dodging attacks. The joke would be she's just bait for a boss/enemy.