clandestine - [klan-des-tin]
1.characterized by, done in, or executed with secrecy or concealment, esp. for purposes of subversion or deception; private or surreptitious: Their clandestine meetings went undiscovered for two years.
2.Kept or done in secret, often in order to conceal an illicit or improper purpose.(

Just by that definition, can you tell why I named a Zero and Yuuki shrine Clandestine?

To me, Zero and Yuuki relationship is forbidden and it's kept a secret as to how they really are with each other. Hince, Zero being a vampire and Yuuki being a human that lets him suck her blood. It even stays that way once you learn of Yuuki being a Pure Blood. It's a relationship that just can't happen between a pureblood and a vampire hunter.

So, to me, the word just suits the two's relationship.