"Harry knew they would hit it off -- she's totally Louis' type," a source recently told Us Weekly.
Even so, it's not always easy for the University of Manchester student to date a guy chased by screaming fans. "They've had tough moments," the source explained, "but Louis is a hardworking guy; he would never give up when things get difficult. He's also extremely loyal."
A pal added, "They'll be together forever. You get that feeling being around them."
Earlier this year, Tomlinson assured Us he'd never break the 20-year-old brunette's heart. "I'm a one-woman man with Eleanor," he said. "I can be a romantic -- but not too sickly."
In a November Seventeen cover story, the "Little Things" singer, 20, explained why Calder is The One.
"Aside from the fact that she's hot, she had everything that I was looking for. She's funny, loud and confident," he said. "I'm not the type of guy who enjoys being single. I'm happy with the way things are with Eleanor. I told her I loved her after five months. I didn't just want up one day and think I was in love -- I knew it in my heart."