The landlord just wants another person to go after in case you default.

The only concern is, when you leased the appt., you probably only told the landlord you would be living there alone. The lease may say something to the effect of, "in the event of a change in occupants, the rent may be adjusted".

Read the lease, but the landlord really has no way to make your GF become a tenant on the lease. It also doesn't matter if the check comes from her, she can be your agent. In the event landlord rejects her check, have her pay you and consolidate funds into 1 check.

You will never get evicted simply because she is living there. A judge will never evict you for this, even if it is a "technical default" of the lease. Just keep paying the rent on time. If you have the lease, fax it to 617-209-2155 and I will review it for you and let you know any potential problems.