1. First of all, this girl's signature is a lock, so I think it is very likely that this girl wants to close herself up temporarily. In other words, this girl doesn't want to have more contact with the outside world for the time being, so she will let others know her temporary state by isolating herself from the outside world. For example, sometimes when we are in a bad mood, we will lock ourselves up alone, so we will have less contact with the outside world. This may actually be the reason why his signature is locked.
Then there is another possibility that the girl's signature is a lock, indicating that he hopes to find someone who can give him the key. Because we know that a lock needs a key, the girl's signature is the number one, and he is actually the one waiting for him. In other words, I hope someone can really like him, find his side and open the key in his heart. This is what he really wants. So this may also be the real idea of this girl's personality signature.
And most importantly, having a lock with personalized signature may only represent his mood at the moment. For example, he tried to do something without success, just like doing something without a key to open it, so he closed it, so maybe he met some difficulties at this moment. Therefore, when this girl meets her own difficulties, she may also set her personal signature as a lock. It may be more appropriate to express his feelings.