Who/what is Seiha?!

My bf got me hooked on the books. I’ve read all the other series and I’m so confused by book 6. Who is Seiha really? Why the facial markings??

Also. Side note. Why did the bromance between characters basically disappear in the last two books? Corin is trapped for potentially years in the time shrine and he gets back to see his friends, zero emotional response from either side?