**Achazyahu** (Ahaziah) son of Yehoram became king over Yehudah at age 22 [(see Yalkut end of Melachim II in the name of Seder Olam)] in the year 3055, the 12th year of Yoram son of Achav (even though Yehoram his father became king in the 5th year of Yoram son of Achav and reigned 8 years which totals 12 years, the Ralbag resolved that the 12th year was not complete so it wasn't counted, and what is written in chapter 9 that in the 11th year of Yoram son of Achav Achazyahu became king, thus it seems it was at least the 12th of Yoram. The 5th of Yoram and the 8th that Yehoram king of Yehudah reigned, Rashi already resolved that Achazyahu reigned one year during his father's lifetime when he was afflicted with severe illnesses (as written in Divrei HaYamim II 21), and what is written in Divrei HaYamim II 22 that Achazyahu was 42 when he became king but all the days of his father Yehoram were only 40 years as written in Melachim 8.
According to this, Achazyahu the son would be older than his father by 2 years, but Rashi in Melachim II 9 in the name of Seder Olam chapter 17 and the Radak and Ralbag resolved this, see there). He reigned one year and did evil and Yehu son of Nimshi killed him [(Tzeda)]. His mother was Atalyah daughter of Omri king of Yisrael [(Melachim II 8:9) and above it says that Atalyah daughter of Achav was Yehoram his father's wife, and it says daughter of Omri because grandchildren are like children, and what it says that he was son-in-law of the house of Achav, he was actually a grandson of the house of Achav which is also called son-in-law, meaning through marriage was his closeness to them (Mikhlal)].
Yehoshafat was the High Priest and Elisha the prophet [(and in Sha'ar HaDorot 18 it says Yehoshua was the High Priest)] making 12 generations [(section 7)].