A dependable, steady person who is responsible and handles things reliably; someone who can be counted on.
田中さんはクラスの中で一番のしっかり者だ。
Tanaka is the most dependable person in the class.
部長はしっかり者だから、任せて安心だ。
The manager is dependable, so it's reassuring to leave things to them.
子どものころから妹はしっかり者で、家事をよく手伝ってくれた。
My younger sister has been steady since childhood and often helped with household chores.