芯が強い

Meaning

1. expadj-i
   mentally strong; strong-willed

Example sentences

• メアリーはおとなしそうに見えて、実は芯が強い子だよね。

  Mary appears quite mild, but in reality she is a strong hearted child.

• トムはああ見えて芯が強いから、トムに任せたら大丈夫だよ。

  Even if he doesn't look it, Tom is quite determined, so it'll be fine if you leave it to him.