The love interest in a story is a very easy character to explain. They are usually close to the protagonist in some way and it is very possible that the love interest and the protagonist will get together by the end of the story. Just keep in mind as you read this post that I am a writer who tries to limit all romantic subplots in my series because they can be highly unnecessary and are usually affected by fan pandering. All I know about the love interest in a story comes from the many nonerotic romances that I have enjoyed watching and reading.
If you are creating a love interest the first thing you should keep in mind is that the love interest should either be very similar to the protagonist or the exact opposite. If they are similar their similarities should be in something like their pasts or health issues. The second thing you should remember is that the love interest and the protagonist should work well together, though they can still fight at times as any pair of people will fight occasionally. Finally it is entirely for two characters to contrast each other in some ways and be similar in other ways.