Adopted logographic Chinese characters (Kanji) in Japanese. One of the omiyages by 空海 brought back to Japan from Tang dynasty in Heian period. Each Kanji has 2 category of readings: 音読み (Chinese) and 訓読み (Japanese)。
In the modern days, there is only 2136 kanjis you need to learn to be proficient. If you learn 6 kanjis a day, that's less than a year to proficiency. Check out WANIKANI (spaced repetition service) and the complimentary app KameSame:
WANIKANI: https://wanikanji.com/
My problem of learning Kanji is different, because I know all of them from Traditional Chinese. They just pronounce differently in Japanese...