My Boyfriend Chose not to Move in as My Roommate

My question involves landlord-tenant law in the State of: WA

My Ex-Boyfriend and I rented a house together. He was the primary signer and I the Co-leaser. The day we moved in he decided to take another path, stayed a few nights and backed out of our arrangement. He said he would continue to pay half of the rent, but has not paid the last two months. This is a six month lease. The lease will be up in 2 months. I, not prepared to pay the whole amount of rent here but had to pay the full amount last month, but am running short. The LL in order to help me out is taking the deposit money to use for this months rent in anticipation of him not paying again (After this there is only one more month due). What are his legal obligations to paying on this lease and does he have rights to get the deposit back (which he put down)?

You have told us that your landlord is already applying the deposit toward unpaid rent, so taking you at your word no deposit is going to be returned to anybody.

If you sue him to try to recover his share of the rent, you will need to establish that he promised to pay the rent despite never moving in and, assuming he claims a failure to mitigate, also that you made a reasonable effort to mitigate your damages (e.g., by seeking an alternative roommate) once you realized that wasn't going to happen.