Said to be a cosignatory to the Corellian Treaty along with Mon Mothma and Bail Organa. This instrument formally began the Rebel Alliance.
The last representative of the Corellian system (which would eventually produce the Millennium Falcon) in the Galactic Senate of the Old Republic, Ibilis was always fiercely independent. He was naturally, therefore, one of the first to balk under Palpatine's Chancellorship. He was an early joiner to the Delegation of 2000.
After the Empire came, he quickly attract a large following of rebels dedicated to restoring the Republic. Eventually seeing the wisdom of adding his followers to those of other charismatic leaders, he became one of the three main signers of the Corellian Treaty along with Mon Mothma and Bail Organa. This instrument formally began the Rebel Alliance.
But it was an uneasy Alliance. He mistrusted Mon Mothma, believing her to be as power hungry as Palpatine. His objections may have had the positive effect of forcing Mothma to put herself up for regular re-election, thereby making the Alliance more of a functional democracy than a loose collection of rebels. When Alderaan was destroyed, taking Organa with it, the Alliance lost its balance somewhat. Organa's role had long been to steer a middle course between Mothma's pacifism and Bel Iblis' more hawkish tendencies.
Shortly after the destruction of the first Death Star, Mothma and Bel Iblis had a huge falling out over missions tactics involving Corellian troops. Mothma, without Organa's hand to stay her, dismissed Iblis from battlefield command. Despite the recent victory over the Empire, Iblis then pulled out of the Alliance entirely, taking Corellia's fleet production facilities with him. This, it has been suggested, is why the Rebellion is in such disarray at the outset of Episode V, even though they seemed to have scored a massive victory at the end of Episode IV.
Though Iblis' forces would continue to harass the Empire throughout the remainder of the CIvil War, his feud with Mothma would remain intact. He formed relationships with other unallied, but Empire-opposed, systems for the remaining years of rebellion. Most notably, he got in bed with the Bothans. The Bothan leadership served as a bridge between Iblis' splinter group and the main Alliance.
Mothma's brave use of the Death Star II intel for which "many Bothans died", was the first stirrings of a thaw between the two former allies. When, after successfully using that intel, the Empire fell and the New Republic rose, Iblis began to reassess his impression of Mothma.
Their rapprochement was speeded by her behavior in the first years of the New Republic. She proved herself, in his eyes, to be a democratic leader after all--while the Bothans seemed to be headed down a more dictatorial path. He then brought his weight suddenly back into the Alliance/New Republic, allowing Mothma to quickly consolidate her victories.
Many credit the rapid New Republic swift real estate gains with Bel Iblis' welcome return to the fold.