Alert on Guard Lingering Mind in Chinese / Japanese...

残心 is a Japanese Kanji word meaning: continued alertness; unrelaxed alertness; remaining on one's guard; lingering mind, and being prepared for a counterstrike. This context is used in martial arts, which is probably why you are looking up this word.

In archery and golf, it can be the follow-through.

In the context of love and relationships, it can be lingering affection, attachment, regret, regrets, or reluctance.