Who is Ba'al from the Old Testament?

From the Holman Illustrated Bible Dictionary:

BAAL (Bāʹ ȧl)

1. Lord of Canaanite religion and seen in the thunderstorms, Baal was worshiped as the god who provided fertility. He proved a great temptation for Israel. “Baal” occurs in the OT as a noun meaning “lord, owner, possessor, or husband,” as a proper noun referring to the supreme god of the Canaanites, and often as the name of a man.
2. According to 1 Chron. 5:5 Baal was a descendant of Reuben, Jacob’s firstborn son, and the father of Beerah. Baal was sent into exile by Tiglath-pileser, king of Assyria. The genealogical accounts of Saul’s family listed in 1 Chron. 9:35–36 indicates that the fourth son of Jehiel was named Baal.

The noun comes from a verb that means to marry or rule over. The verb form occurs in the Hebrew text 29 times, whereas the noun occurs 166 times. The noun appears in a number of compound forms which are proper names for locations where Canaanite deities were worshiped, such as Baal-peor (Num. 25:5; Deut. 4:3; Ps. 106:28; Hos. 9:10), Baal-hermon (Judg. 3:3; 1 Chron. 5:23), and Baal-gad (Josh. 11:17; 12:7; 13:5). See Canaan.

Brand, C., Draper, C., England, A., Bond, S., Clendenen, E. R., Butler, T. C., & Latta, B. (2003). Holman Illustrated Bible Dictionary (151–152). Nashville, TN: Holman Bible Publishers.

The basic idea is that Ba'al was a fertility god who showed up all over Caanan. Pagan gods tend to take on different characteristics and different stories, especially when popular, and so it differentiate them, sometimes specific place names are added.

That Baal is sometimes combined with YHWH stems more from the fact that the Israelites themselves became highly syncretic, moreso than any firm sociological study that says they stem from the same place.

Baal and Ancient Israel: The cult of Baal was widespread in the Syro-Palestinian world and became the focus of Israelite religious animosity. Baal’s consort in Palestine was not Anat, but Asherah (Judg. 3:7) or Astarte (Judg. 2:13; 10:6). Syncretism had blurred distinctions between Asherah, Astarte, and Anat, while for Israelite writers such distinctions were not of interest. We hear of the cult of Baal in a number of local manifestations: Baal of the Covenant at Shechem (Judg. 9:4); Baal of Peor at Shittim (Num. 25:3); Baal ‘Zebub’ (‘of the flies’; but should Zebul, ‘Prince,’ be read?) of Philistia (2 Kings 1:2-3); and perhaps Baal of Hamon (Song of Sol. 8:11). Jezebel introduced to Samaria the worship of Tyre’s god Baal (1 Kings 18:19). It is not altogether clear whether these local baals were taken to manifest the single great god Baal or whether they were imagined as discrete deities.

Achtemeier, P. J., Harper & Row, P., & Society of Biblical Literature. (1985). Harper's Bible dictionary (1st ed.) (84). San Francisco: Harper & Row.

There have been people who promulgated the theory that Baal and YHWH were the male and female essences of the same pre-caananite god, but this is by no means a universal interpretation.

The relationship between El and Baal in Canaanite mythology has been a matter of dispute. There is some indirect evidence of antagonism between these important gods, inasmuch as they were competitors for the highest position in the pantheon. Yet there is also evidence of concord between them. Philo of Byblos reported an accommodation whereby Baal ruled on earth with the permission of El; many have seen in this arrangement the pattern of relations between the two most important gods of the Canaanites.

Achtemeier, P. J., Harper & Row, P., & Society of Biblical Literature. (1985). Harper's Bible dictionary (1st ed.) (84). San Francisco: Harper & Row.

Interestingly, by the time of Jesus, Baal had morphed into "Beelzebub", a name commonly associated with the devil.

In the Ugaritic epic material, Baal is pictured as descending into the netherworld, the domain of the god Mot. That descent was evidently part of a cycle intended to coincide with the cycle of seasons. In order to bring Baal up from the realm of Mot and thus ensure initiation of the fertile rainy season, the Canaanites engaged in orgiastic worship that included human sacrifice as well as sexual rites (Jer 7:31; 19:4–6). Sacred prostitutes evidently participated in the autumnal religious ritual. The worship of Baal was strongly condemned in the OT (Jgs 2:12–14; 3:7–8; Jer 19). See also Canaanite Deities and Religion.

By NT times, the name had changed to Beelzebul (KJV Beelzebub), from the Syriac language meaning “lord of dung.” It was a common practice to apply the names of the gods of enemy nations to the devils of one’s own religion. Thus, the title was applied by the Jews to the devil, or Satan, the prince of demons (Mt 12:24, 27).

Elwell, W. A., & Comfort, P. W. (2001). Tyndale Bible dictionary. Tyndale reference library (135). Wheaton, Ill.: Tyndale House Publishers.