one-letter kanji of love: koi

The English word "love" has two meanings in Japanese. Each has a distinct Kanji symbol. One of them is "koi," which mainly refers to "falling or being in love." It has other meanings, but in most cases it's used for love between men and women.

Strangely enough, you don't use koi for married couples. There's a virtual proverb which goes, "When a man and a woman get married, koi turns into ai." And the ai is the other kanji character of love.