This series holds a special place in my heart; it’s the one that got me learning Japanese! Quick summary from Amazon:
“Rakan, an orphan with no knowledge of his past, meets Chigusa, a mysterious man who tells him he is a Sanome, someone who can control how plants grow, as they battle against assassins and monsters from another world.”
The manga’s got a great sense of humor with a plot that slowly builds into a very satisfying conclusion. The mangaka’s the same as the one who wrote 氷の魔物の物語, for any who’ve read that.