WARNING! This will include spoilers with evidence through the whole series. Edit
This also includes opinions... EditPlease do not fight over who is better just simply state what you think and be done with it. Please and thank you!
Sophitz or Fitzphie is what Sophie and Fitz are called as a team by Keefe. Sophie has had a crush on Fitz through the whole series and has second thoughts once in awhile. Fitz is the first elf Sophie had met and he's the one who showed her where she really belonged. There was only one time when they fell apart and that was in Exile when Alden's (Fitz's dad) mind became broken. Fitz is the one with the closest relationship with Sophie because they are cognates-in-training with multiple trust exercises which includes telling all of their secrets to each other. Fitz only started really showing his possible feelings for Sophie in Lodestar when he holds hands with her often but also leans in and almost kisses her but Keefe interrupts. This is another possible match... read below.
Keefe is always the first one to volunteer for anything that involves Sophie including a time in Neverseen when Dex makes a breathing-underwater contraption for two, and he wants to share with Sophie (If they had shared, their faces would be extremely close together, their noses touching). Keefe also insists in being by Sophie and sometimes says things that also may seem flirtatious. He always says out loud in school multiple times, "It's a date, Foster!" sarcastically, but to show off when he comes over. He also always insists on her being more relaxed and calm. It never really says she likes him but sometimes she gets the weird heart fluttery thing and also cares a lot about Keefe. Another thing was on Neverseen when Keefe and Sophie are by the mountains in the Sanctuary when it says, "...her stomach filled with fluttering things, which flitted around even more when she noticed how close they were now standing. The toes of their boots were almost touching, and his breath felt warm on her cheeks."
The two hold hands a lot, especially in NEVERSEEN, when Sophie seems to be the only one of the cast of characters really helping Keefe get through his hard times in that book. Keefe has also called her "cute" multiple times. During LODESTAR, both are there for each other during tough times: The Neverseen, Bad Memories, Mr. Forkles Death, and finding Sophie's missing family.
Dexphie is a mix of Dex and Sophie and can possibly be a love match. Dex and Sophie have been best friends through the whole series and have a close relationship. Dex has always made cool contraptions for Sophie to keep her safe including the Sucker Puncher (gives a burst of air so then your arm goes at a faster speed punching harder.) and a panic switch (a ring with with a switch that will alert Dex to come help.). Dex was the "lucky" person and was going to see Sophie in the caves when the Neverseen came and kidnapped both of them doing horrible things like drugging and burning them. Dex always blushes with Sophie and takes any moments he can to be with her. He is always jealous with Fitz and how they always are with each other. In NIGHTFALL it's obvious Dex likes Sophie, but when he gets her some "crush cuffs" as a cover up to hide her Enhancing ability, Sophie kisses him. She explains to him that there was no spark to the kiss, and they agree they are just friends.
Tam and Linh are also mentioned in Neverseen, as they help when Sophie and friends go to Ravagog. In Lodestar, Tam also is mentioned several other times in relation to Biana, including when Keefe makes clear that he is the president of the Foster Fan Club and Tam says he has no problem with that and looks at Biana, blushing.
Keefiana is the ship with Keefe and Biana. In the first book, Biana is jealous when Keefe calls Sophie for his base quest team. In Everblaze when Lady Gisela points a melder to Biana's head, Biana acts brave, though Lady Gisela claims it was "mock-bravery" to impress Keefe. In Neverseen, Keefe reveals he kissed Biana on a dare, and Biana is holding Keefe's arm on the cover.