Who, which, that, the one who. He who began a good work in you will carry it on to completion until the day of Christ Jesus. God finishes what He starts.

to tower up (be conspicuous), i.e. (by implication) to be suitable or proper (third person singular present indicative, often used impersonally, it is fit or right)

Sound health, uncorrupt, true in doctrine. Hygiaínō. Paul urges Timothy to hold to sound words and sound faith. Bodily and doctrinal health are both marks of a life aligned with truth. Guard what you believe as carefully as what you eat.

Teaching, instruction, doctrine. Didaskalia. All Scripture is profitable for doctrine. Sound doctrine is the skeleton of the body. Without it everything collapses.