Bikini Angel
This takes place In High-School DxD: Light Novel Volume 23 - Upsetting sorcerer 2. In this, Issei's team is having a rating game with that of Dulio Gesualdo's. And Dulio under the guidance of Rudiger Rosenkreutz, played a trick on Issei.
He trapped him in a bubble where a recorded image appeared. On the recording we have Dulio begging Gabriel, one Four Great Seraphs, to do something. She then complains about how embarrassing it is. She couldn't believe she was dressed the way she was as it was so shameful.
But Dulio continued to pressure her by saying this will help motivate the Angel team, too. Everyone outside the bubble could hear what was recorded, but only Issei could actually see it. And what he saw and yelled that he saw was Gabriel in a bikini, to which he had a nose bleed.
The bikini in question was white with a stringed bottom and stringed shoulders. But more importantly, it was very tiny. The bottom showed off a bit of her crotch lining. And as for the top, it was barely able to cover her large breasts as all it did was cover the nipples. And it didn't even do a good job there was some pink sticking out the top. We can even see that the right shoulder strap slipped off her said shoulder.
As she's being recorded, she while blushing asks if she really has to wear this. Dulio tells her yes, because her wearing this bikini will save the Brave Saints. So she must become their weapon. Despite this, she still complains about the near non-existence of the fabric.
Dulio once again asks her to please wear it. Despite her complaint a second ago, she said "Uuh, yes…I’ll wear it." in response. After that, Issei starts screaming in excitement. In fact, he was too excited to listen to Ravel who was trying to warn him.
It's then we learn that it's not just Gabriel. It was a recording of multiple Angel women wearing a number of things with her. Gabriel and the Angel women were also wearing school swimsuits, transparent negligee, and athletics clothing. All of which were working their magic on Issei.
Things to take note of:
1 - Akeno claimed this tactic was extra effective because Issei always wanted to see Gabriel's breasts. But truth be told, he had seen them prior to this when he used his new Penetrate ability to see everyone, including her naked. This would mean no one actually knows he saw.
2 - Despite being an Angel, even a Great Seraph, the image on this page is the only illustrated image of Gabriel in the Light Novels. Hell, even counting other media, it's the only time we actually see her at all.
