One of the primary goals in Fire Emblem Heroes is building up a squad of characters that you're interested in. Unfortunately, many times you're stuck with unattractive heroes that you aren't interested in. That's where reroll comes into play.
If you want to get the hero you want, there's a trick. It's not a pretty one, and it requires uninstalling the game, but it works.
1. Complete the first 3 missions at which point you should have 20+ Orbs.
2. Use multi-summon.
3. See if you get the hero you want. If not, uninstall the game and repeat Step 2.
Note: Don't link your Nintendo account to the game or this method won't work.
Check Out More Fire Emblem Heroes Coverage On GameRevolution:

Embark on an inspiring mission of human connection beyond the UCA. Sam — with companions by his side — sets out on a new journey to save humanity from extinction. Join them as they traverse a world beset by otherworldly enemies, obstacles and a haunting question: should we have connected?