Short Guide on How to Unlock the Secret Character Gains Boros in Vampire Survivors.
To unlock Gains Boros, first enter the stage The Bone Zone. It is highly recommended to apply Hyper Mode and choose a high-movement-speed character, such as Red Death, as you will need to do quite a bit of walking, and the movement speed bonus will be greatly appreciated.
Once inside the stage, start walking north. Just beyond the Silver Ring stage item, you will find a big ring of flowers that will periodically heal your character if you stay inside of it. Remain inside the ring of flowers for 10 seconds, and the ring will disappear, followed by a short jingle signaling that you have unlocked Gains Boros.
If you prefer, you can unlock Gains using a cheat code. Simply access the Secrets menu, which you can find in the main menu if you've unlocked The Forbidden Scrolls Of Morbane. Then, enter the code 'highfive' and you will have Gains unlocked.