Sincere
Honest, Faithful
noun, な adjective
You have sincerity and you have truth. You are so sincere and honest and faithful, good friend.
せいじつ
This is a jukugo word that uses the on'yomi readings of the kanji. You should be able to read this on your own.
彼は僕の誠実な友人だから、心配する必要は無いよ。絶対に取引から手を引いたりなんかしないさ。
He is my faithful friend, so don’t worry. He will never back out of the deal.