While Lü Bu dies early on in history, I'm not sure it's entirely fair to dismiss him as a "minor figure" - he is of at least moderate importance. Historically, as noted by the Sanguozhi, he did two quite important things:

• He betrayed Ding Yuan and pledged fealty to Dong Zhuo, part of what was allowing Dong Zhuo to occupy the power vacuum created by the strife in Luoyang.

• He assassinated Dong Zhuo, perhaps in collaboration with Wang Yun, as the coalition was crumbling, and the aftermath of that was a major part of what enabled Cao Cao's rise to become the most powerful man in China.

After this, of course, Lü Bu basically becomes just another one of a bunch of minor warlords, and because of his notorious duplicity, he is eventually mopped up by the ascendant Cao Cao after the elimination of Yuan Shu as a major threat (Lü Bu, fool that he was, defected to Yuan Shu after the latter had been more or less reduced to the status of a minor warlord).

As we go further from the Sanguozhi itself and into its commentaries, we quickly run into antecedents of a mythologization of Lü Bu. While the Sanguozhi has Lü Bu being an exceptionally skilled archer and a notoriously strong man, the commentaries of Pei Songzhi adds the classic 8 or 10 character saying,

人中有呂布,

馬中有赤菟

roughly translating to

Paragon of men: Lü Bu

Paragon of horses: Red Hare [the horse of Lü Bu]

But the general theme of Lü Bu's character is evident already within the Sanguozhi and provides the backing for the literary figure - he is a courageous man of tremendous martial virtue, but also rash, impulsive and subject to his personal desires, which led to disorder and corruption among the men under his command. So he possessed a kind of powerful, raw masculine force that was untempered by other virtues, ultimately making him a deeply flawed character.

I think it is already obvious why Lü Bu is an interesting character to anyone who wishes to write an epic - he represents the zenith of a certain kind of virtue (martial) and the nadir of many others (wisdom, honesty, loyalty). And in the storytelling tradition epitomized by the Romance of the Three Kingdoms, this is the Lü Bu we learn of. His betrayals are emphasized (he is made the adoptive son of not only Dong Zhuo, but also Ding Yuan whom he betrayed originally), and his unmatched prowess as a warrior is put front and centre. Indeed, he simultaneously duels Guan Yu, Zhang Fei and Liu Bei to a standstill - with Guan Yu being portrayed as the second greatest warrior of the entire epic, and Zhang Fei being among the runner-ups for third greatest.

Yet, it is Guan Yu who is celebrated as a "god of war", and not Lü Bu. Why is this? Obviously, Guan Yu goes on to live for several decades and has many interesting anecdotes attached to him. Yet, there's more to it than that. Lü Bu is driven by a kind of burning passion and desire, a want for greatness, seizing every opportunity to demonstrate his manliness. Guan Yu, on the other hand, despite his nearly-unmatched prowess, is driven by a sense of loyalty, a sense of concern for his friends, and a desire to maintain his honor. In the Romance, one of the scenes that drives this notion and source of internal conflict home is the encounter at Huarong Pass, where Guan Yu decides to allow the defeated Cao Cao to pass safely, in recognition of their earlier friendship and the hospitality with which Guan Yu had been treated in Xuchang.

There's an interesting quality to Guan Yu, where he is - in stark contrast to Cao Cao and Lü Bu - portrayed as of a more or less chaste mind, in line with his general temperance - the one exception being the dream he has shortly before his death, which can be interpreted as a subliminal desire to overthrow his master and sworn brother, Liu Bei, and take his two wives (Edit: But see discussion below with /u/Dongzhou3kingdoms). But this generally chaste mindset portrays an ideal of the yingxiong or "hero", that was popular at the time of the composition of the Romance, that can broadly be said to unify notions of Confucianism with notions of Buddhism. And it is in being consumed by desire and personal ambition, discarding loyalty, that Lü Bu ultimately fails the test at being a true Yingxiong.

Some sources and further reading:

Sanguozhi (Records of the Three Kingdoms) with commentaries

Sanguo Yanyi (Romance of the Three Kingdoms)

Sexuality, Masculinity and Politics in Chinese Culture: The case of the 'Sanguo' Hero Guan Yu by Kam Louie

I can't comment on Lu Bu's popularity in China, hopefully, this adds to /u/lcnielsen answer. Part 1 history, part 2 novel

History

Was Lu Bu a minor figure? While the slaying of Ding Yuan made Dong Zhuo's rise easier rather then game-changer, killing Dong Zhuo (or being the plot that pulled it off) did have repercussions as mentioned. As a guest general of Yuan Shao, he played his part in the breaking the power of the Black Mountain Bandits under Zhang Yan. As a warlord, he forced Cao Cao back to Yan and came so close to victory that Cheng Yu had to have a go at Cao Cao to stop him submitting, the revolt saw Yuan Shao possibly having to reinforce Cao Cao and tied up troops fighting Zang Hong's revolt that resulted from Zhang Chao's death. When he fled to Xu, he shaped events there via his erratic behaviour, he took it from Liu Bei who would struggle to find a long term base for some time after that, he broke Yuan Shu's power north of the Huai while humiliating the new dynasty.

Now in a civil war from 190-280, he was only in it for 8 years and only an independent warlord for four. That is a short time frame and plenty of longer-lived figures to focus on but ut his story intersects heavily with two the kingdoms that would be the three kingdoms, with the man blamed for the civil war starting and with the Yuan clan.

In terms of achievements, as a military leader, he had his share of victories like the ones I mentioned above. He was able to foresee Yuan Shao's attempt to kill him and had music played inside his tent as a decoy while he hid outside at night, he was able to foil Yuan Shu's schemes for Xu. He also had to run with his wife in a state of undress through his city when Yuan Shao and officer Chen Gong got Hao Meng to try a night time coup.

Then there are his failings, his constant betrayals and inability to stick with an alliance that make him so unreliable and which are a major part of his reputation. His failure to think things through or think long term, his womanizing, his inability to impose himself on his split officers and organize them properly.

Lu Bu's personality is fairly well set out (both the negative and positive) for someone not of the three kingdoms themselves. A womanizer, a bully, unable to impose himself, short term thinking but also a man capable of a clever scheme in a moment, a man who offered his life to his servants at the end, who could take criticism on the chin and make oath friends.

The appeal as has been mentioned is his strength. Few warlords were noted for it and Lu Bu even among warriors was considered highly. It was his skills that caught Ding Yuan's attention, Chen Shou's SGZ notes (as well as the Wu's Caomanzhuan quoted by u///) "Bù was adept with bow and horse, his bodily strength surpassed other men, and he was called Fēi-Jiāng “Flying General.”, his bravery and skill in the field (as well as unreliability) that saw Xun You urge Cao Cao to deal with Lu Bu early.

We get examples of his skill, the ability to dodge out of the way of Dong Zhuo's throwing axe, Wang Can's Yīngxióngjì has Lu Bu winning a rare duel with Guo Si, none of Yuan Shao's officers dared to pursue Lu Bu after failed assassination attempt. The archery feat to end a war between Yuan Shu and Liu Bei is possibly the most famous

Bù ordered a gate attendant to at the center of the camp gate set up a halberd. Bù said: “Gentlemen, watch me shoot the halberd’s small blade. If I make this shot you gentlemen shall withdraw, but if I miss you may stay and settle your quarrel.”

Bù lifted his bow and shot the halberd, right in the center of the small blade. The officers were all startled and said, “General, you indeed have the authority of Heaven!” The next day they again had a celebration, and afterward each side withdrew.

Lu Bu's army might also have helped put Yuan Shu's commander Ji Ling off but the focus is on the archery diplomacy