Reze has more chemistry with Denji because that's her role, a love interest for Denji no more no less. Asa on the other hand is clearly not supposed to be just another love interest. Her relationship with Denji is never depicted as sth really that important. Her issue with Yoru is always her main conflict, not Denji. Her saving Denji is no more special than her saving Yuko, that's just meant to show how selfless, how heroic she is despite having a savage devil constantly on her throat.
And she's not boring. It's her relationship with Denji that was boring because Denji is not a compelling love interest for her. Every 2-sec interactions of her with Yoshida always had 10x more chemistry and much more intriguing.