Kagura was pretty sure that her soulmate was a Yato, and a bloodthirsty one too. The demented looking rabbit with demonic red eyes tattooed on her skin was a no-brainer for her.
She didn't want to reveal this to anyone even her family because this was the only thing that was truly hers. Although it was selfish of her to think that, Kagura needed to have something like that. That was why she guarded her soulmark like a dragon guarded its hoard.
She could also use the excuse that her family's non-mentioned 'don't show your soulmark' rule influenced her as well. Her parents and Kamui never showed her or anyone their marks.
Although Kamui never showed her his mark, Kagura unintentionally managed to get a glimpse of it. It was when Kamui got into another fight and she was helping bandage his wounds. When her brother took off his shirt, a flash of white in the shape of a rabbit with what seemed to be two dots of red briefly showed before Kamui shifted subtly. The casual and almost undetectable movement hid the soulmark from her.
As bad as she should have felt, Kagura felt giddy at the glimpse of her brother's tattoo. She and Kamui had similar tattoos. The young Yato was 5 years old at the time and was very childish. This is why she decided to ask her brother if he had the same soulmark as her. She showed him her mark by revealing her exposed wrist to him.
A stoic Kamui who stated that his soulmark had an umbrella in it came out of this action. Kagura felt disappointed, but she knew that she and Kamui would be bad soulmates to each other.
Her brother was always distant in a way to her. A cynical part of her would always whisper to her that she was just a burden to him and their mother. Tears would bead in her eyes whenever she thought of this.
When Kamui left, Kagura would banish the thought away from her mind. She shouldn't think about that traitor! He abandoned her without a second thought. Sure, if they ever crossed paths she may try to bring him back. But what then? What was there to go back to? Their family was broken. Her father was off hunting aliens across the universe, her mother was dead, and Kamui was off doing who knows what-!
For the first time, Kagura gave up on something she was determined to recover. After all, how can you fix something that would just continue breaking?
As she thought this, Kagura could've sworn that a pulse of warmth came from her soulmark.