Seiren is Kaname's unofficial bodyguard (reference: look at chapter 7). Seiren works for Kaname. They are also night class students which means Seiren is a vampire too, but she is not a pureblood. It is most likely she is an aristocrat, but could possibly be a common vampire.
Seiren is often seen with Kaname, thus suggesting that the two do indeed have a close bond as Kaname doesn't bond easily with others. Kaname, in chapter 72 (raw only) has been seen with Seiren even after his murder of another pureblood vampire. This has suggested to many fans that Seiren may be close to Kaname, whether they are friends or share a deeper relationship (in the past or at present). However, nothing has been proven, and their relationship is speculation for the most part. Nevertheless, they have been seen together on more than one occasion, and it is apparent that Kaname trusts her deeply. Seiren has proved her loyalty to Kaname many times, risking her life to spy for him and protecting the Day Class students alongside Akatsuki Kain and Ruka Souen.