By the end, it’s especially clear that Fifteen is a supporting character.
It’s a quick line but holds a lot of meaning.
“Incel” is a contraction of “involuntary celebate,“or sometimes “involuntary celibacy.”
It can often make them physically and/or emotionally abusive toward women.
Put eloquently by the Doctor, Belinda “needed a Time Lord to take the kaboom.”
In other words, he absorbed the energy that would have killed Belinda as it did Alan.
So, the mysterious connection between Belinda and the Doctor is strengthened even more by this event.
However,Davies adds a sharp left turn at the end of the episode.