心残り (kokoronokori: regret; reluctance) heart + to remain
私は高橋さんに一度も優しくしてやらなかったのが心残りだ。
Watashi wa Takahashi-san ni ichidomo yasashiku shite yaranakatta no ga kokoronokori da.
I regret that I have never been kind to Takahashi.私 (watashi: I)
高橋 (Takahashi: family name) tall + bridge
一度も (ichidomo: never (in a sentence with a negative verb))
優 (yasa(shii): kind)