Relic Hunter Zero is a free to play open source game.

You start with 2 characters and can unlock 4 others through gameplay.
The game consists of 4 areas with 3 stages each followed by a checkpoint and then a boss battle.
“Bounty” you collect from defeating enemies remains through all runs and can be used to by items from checkpoints to permanently add them to your starting base for later runs.
The 8 relics in the game are each broken into 3 parts. They can be found at the end of a stage randomly or bought from a checkpoint. Only 3 relics can be active at the same time.