I'm fine

It's not like you wouldn't feel pain, you just can't afford to be slowed down by it, so why should anyone need to know?

Level 3: I'm fine, realy

You have Advantage on deception checks.

Level 3: I'm fine, i won't slow you down

You are immune against movement speed reduction.

Level 7: I'm fine, you can have it

Whenever an ally regains Hit Points, you regains half of the amount they healed.

Level 10: I'm fine, no need to worry

Your unrelenting will to go on inspires others. During combat, you can use your Bonus Action to give an ally Heroic Inspiration whenever they start their turn without it.

Level 15: I'm fine, i'll catch up

You can ignore the first three points of exaustion.

You have advantage on saves against effects that prevent you from moving, such as petrified or restrained.

Level 18: I'm fine, after all i have to

If you feel yourself responsible for something, like a promise you gave or simply because you know you have to because you know that if you don't do it, no one will, you can even defy death. While acting under the named conditions are no longer unconscios, you can take actions as usual but continue to make death saves. When healed, you go on as usual, if succed your three death saves, you can continue fighting but fall unconscios once danger is gone, if you fail your saves, your muscle memory kicks in and you attack the nearest enemy as long as your body can physically move or no enemy is left, then you die, body broken, way to much as a simple revivify spell could safe you, the only way to be resurected is by a wish spell or the true resurection spell.