The Other Darrin: Was initially voiced by Michiko Neya in a one-line cameo in an early Axis Powers episode of the anime, but was voiced by Hekiru Shiina for her more prominent role in an episode of The Beautiful World.
Reincarnation Friendship: In volume 4, France gets to meet and befriend a young girl named Lisa... who is implied to be Joan's reincarnation. A translation cam be found here, and it's clear from France's statement "When I first saw you, I thought God was pulling a cruel trick on me, but I see you're living a happy life. It seems like my wish has been granted," that he cared for her deeply at the very least.
Tsundere: Her usual fanon depiction is this, based off the one line she had, addressed to France: "Don't get me wrong; I'm not doing this for you!"
When Old Fritz was around he would always praise how I could do anything well...
Forgotten Fallen Friend: Completly and utterly Inverted. While Prussia is the most self-centered character west of Korea, he keeps remembering Fritz for the rest of his life and stays emotionally invested in him in a way no other nation-tan is in a human, France and Jeanne perhaps aside. Justified Trope, as the more or less artifical Prussia's nationalism had to be created from scrap and was greatly built on Friedrich the Great and his victorious wars.
Guardian Angel: Prussia believes he is "kindly looking down on me from Heaven". Once he in a sort of prayer-like manner asked the long gone Fritz to watch over his "masterplan", and at other point when preparing to go to sleep again asked him to watch over him, almost as vespers.