in my opinion reiner and bertholdt never labeled their relationship and probably didnt really talk about it much at all. they didn't develop crushes on eachother and have the akward period of "does he like me, what if this ruins everything"
it was so gradual that it almost felt like nothing between them ever changed, but by the end of it they were so different from how they started. things between them started shifting from just being warriors the moment they stepped foot on paradis.
it was them being all alone- but with eachother. it was them only ever being able to confide the whole truth in eachother. it was them being able to share eye contact when in groups, when the other cadets would say something about inacting their revenge, it was their way of letting the other know that they are not alone in this, that they have eachother. it was that bertholdt was the only one who could know about reiners personality disorder, and the trust they had to have. it was at night, when negitive thoughts run fast and high, how they were able to take eachothers hands and sleep knowing that they arent hated by everybody. feel the warmth and know that they are loved by at least this one. and eventually that's what it was.
they loved eachother because they understood eachother in a way that nobody else ever could. they werent just seeking sanctuary in the other out of necessity, but because they mutually just loved eachother. they shifted from warriors to lovers and they never had to question it, or think about it at all. thats just what they were.