I have a companion, an enemy with an ability, and the Reaper in play. Does the companion block the enemy’s ability, then get eliminated by the Reaper?

The Reaper’s effect is applied, and enemy abilities are redirected to the player without a companion. (Let’s say you first apply all enemy effects to your companion, then finally the Reaper’s effect eliminates your companion… this order of application will still redirect all enemy effects to the player.)