I don't think Athena is worse for Billy primarily because of her fireballs... That isn't to say they don't pose a threat to him, because well from the start of the match, he has to reach her... Sure, there's j.C that can "snipe" her attacks, but it's at the risk of trading hits, and/or getting hit by something bigger... She controls the air and can lock him down without real fear of anything-- unless Billy has meter for EX dp... Other than that, crouching C won't do any good against most of her main attacks...
Out the gate, he needs to take that momentum and potential zoning away from her, while possibly baiting either a command grab, dp (especially on wake up), and her super (possibly on wake up, too)...
Billy can't destroy fireballs with his Senpuukon, or his hcf+P specials...
Kensou shouldn't be too hard, as his aerial attacks aren't stronger than Billy's... You can bait a whiff qcf, hcb+P, as well as his rdp+K, since most times you have to be up close for those to hit... If you're playing against a Kensou that loves to hit confirm into rdp+K, then you're probably not gonna see him do one as an AA, unless he just specializes in that move primarily... Even with his fireball setups, you're more than likely are gonna stop his momentum with f+A at some point, in which he'll have to change up his strategy... Crouching C can beat out his air normals most of the time, so once you get that, go into f+A to close in...