Ensemble Dark Horse / Attack on Titan

• Rico's expanded role in the anime has earned her a fairly respectable number of fans, especially after she sliced a Titan's eye open. She's so well liked that she even appears in the final chapter after going unaccounted for in the time skip.
• The "Kawaii Titan◊". People keep making it out to be a marvellous, beautiful, innocent creature that had its life horribly cut short by Mikasa in the Warehouse battle, despite the fact that it was responsible for killing Mina Carolina and came scarily close to killing Sasha as well.
• Mina is a simple Mauve Shirt with only about a page's worth of dialogue before she's munched by the Kawaii Titan during the squad's ill-fated First Battle of Trost, but she's got a surprising amount of attention from the fandom.
• Eren's squadmate from Episode 5, Mylius Zeramusky is gaining quite the fanbase on tumblr as "Background-Chan" despite only having a few minutes of screentime.
• Marlo and Hitch (and, though less remembered, Boris), Annie's squadmates in the Military Police Brigade, have a growing fanbase that refers to them as the "Unicops".
• Levi's Special Ops Squad have all gained a lot of popularity and respect, particularly after they died.
• Marco Bodt. Even though he's the series' first Sacrificial Lion at the end of the first major arc, as well as the least developed of the "important" 104th trainee squad, we still get plenty on his character, such as his Nice Guy tendencies and The Heart status as well as apparently being a natural-born leader.
• Mike for being reliably badass while having several endearing personality quirks such as his superhuman sense of smell. Many readers were quite upset when he died and even more upset when the manga didn't even acknowledge his death for almost forty chapters.
• The horse that attacked Gabi in Chapter 109 became an overnight sensation among the fandom mainly due to the hilarious troll-face it sported afterwards.
• The various Titan Shifters of the past that appear in the final arc, due to their unique designs. Of all, the most popular may be the "okapi" Titan, for 1) swallowing Armin with its long tongue and 2) making Mikasa (and the audience) wonder what the hell an okapi is.
• Onyakopon became significantly popular after his appearance in Season 4, due to his lack of bias against Eldians, his blasé way of handling Sasha's question about his skin, general Nice Guy attitude, and his assistance in helping the anti-Yeagerists, going against Zeke's and Eren's factions and to free them. He's even earned the popular—albeit comedic in nature—nickname "Onion Coupon".