essentially Ghirahim decides he wants to serve only himself and sets out to break free from Demise's ownership. which he eventually does, but by the time it happens, Tesni has become so important to him that he later asks her to claim him as her sword instead, which is a conscious choice he's now able to make himself. the difference between their partnership and the one he had with Demise is that he and Tesni see each other as equals and want to share power together instead of one of them having control over the other, so it's a big display of trust for both of them...and it's also kind of like an extremely intimate marriage proposal sdkjfg