If you have trust in your horse, and became friends, you would not need that helmet
WHAT???? NO NO NO NO NO!!!!!!
REGARDLESS of how long you have known your horse, how much "trust" you have between you and the horse THEY ARE UNPREDICATBLE ANIMALS.
RIDING IS DANGEROUS.
WEAR A HELMET.
I have been riding for most of my life, and I ALWAYS wear a helmet, even if I'm riding some kid's gentle pony. Why? Because you can never be 100% sure what you horse is going to do. Something could happen, even as simple as the horse "popping" a fence or spooking at something - anything - and you could whack your head against their neck (concussion - believe me, been there, done that) or fall.
Even while grooming, it's a good idea. I never used to until just recently: a good friend of mine (an INTERNATIONAL dressage rider) just got seriously hurt because she was grooming the horse's legs, the horse spooked and whacked her on the head with its knee... she got knocked out and hasn't been able to ride since (happened about a month ago). If you seriously think you and your horse have such a bond that enables you to not wear a helmet, get your head checked,
I think it's already damaged.