4. Rose Tyler, 10th Doctor's Companion
I know there are people out there who seem to not like Rose at all, but she really was very loyal to The Doctor. She truly loved him and wanted to stay with him forever. Even picked him or her family. The Doctor showed her a life worth living. She felt so happy all the time, even in the face of danger she knew The Doctor would save her. Although, in the end her fate would was pretty tragic and she ended up in a parallel universe with her family. She did get to be with the Meta-Crisis Doctor in the end, so in a way she did end up having The Doctor with her. Rose seems like one of those friends who would be by your side through anything.