I hope I'm not replying too late... I just browsed into this site's forum for the first time. What you asked for: Endurance 忍 Faith 信 Mercy 慈
There are many ways to express these words, including mostly using more than two characters to form the words. Since you wanted only one character, I translated them the best I could, and picked what I thought the most suitable, and most commonly used in expressing abstract meanings in Chinese calligraphy art. I'm guessing the "true feelings" you have for your friend are actually lover's affection rather than friendship, so although I don't know why you didn't consider the character "Love" ("爱"), I would suggest, in this case, the next best in these three that you've chosen would be "信".