Sarvente was rejected by God from entering into heaven. There is no official reason why she was rejected, but it can be inferred that she made one big mistake that she has done in her life to the point where it didn't matter how good and kind all of her actions were. After experiencing that, she doesn't want anyone to experience what she went through so she will guide everyone to a better place. Now Sarvente's purpose is to guide people to heaven, regardless if she was rejected by God. Sarvente is a devout believer that Heaven is a good place, even if she was dropped from heaven by God's rejection.
She eventually found Ruv again years later hiding at the church after running away. Because Ruv and Sarvente do not remember each other due to Ruv being chased all the time, their first time interacting started out rough. They even resorted to violence through fights and non-stop arguments, but they eventually grew closer after a few years due to Sarvente not showing any signs of betrayal against Ruv. Their friendship improves the longer they lived together.
Sarvente sees good in Ruv and believes that he can go to heaven which is the primary reason why she does not show any signs of betrayal against him.