Wow — talking about romance in 'Durarara!!' always feels like walking through Ikebukuro: messy, surprising, and full of little lights. The clearest romantic thread that the series actually leans into is Celty Sturluson and Shinra Kishitani. Shinra is openly smitten from early on, and Celty returns that affection in her own quiet, tangible way; they live together and their bond is the most straightforward, long-running coupling shown in both the anime and the novels.
Beyond that, there's Masaomi Kida and Saki Mikajima — they have a real history. Saki was Masaomi's girlfriend/childhood sweetheart and their relationship is a major emotional anchor for his arc; it’s complicated by his past, but their feelings for one another are treated as genuine rather than purely plot fodder.
Then you get the softer, thornier stuff: Mikado Ryugamine and Anri Sonohara carry a lot of romantic tension. Mikado clearly crushes on Anri, and Anri cares deeply for him; whether it’s full-on romance or a more protective, intertwined friendship depends on how you read the ending of 'Durarara!!' and how much weight you give the subtler cues. After that, many connections are one-sided or ambiguous — obsessions, loyalties, and friendships that look like romance but aren’t always labeled as such. For me the emotional core is what matters: Celty & Shinra feel like a real couple, Masaomi & Saki are a tragic-turned-hopeful thread, and Mikado & Anri are quietly tender and unresolved — which I kind of adore.