Shiro and the gang successfully escaped the Autumn island and returned to winter island. Even though they got back safely their pride took a hit. Shiro was a prideful person even though he rarely shows it, he indulges in the sin of pride. While Ichie and Akira didn't suffer too much but they now realize that there's always someone stronger than you. They felt useless when Shiro fights the beast and they weren't even sure they could defeat Iris.
Shiro then realizes, even though DxD anime shows that this is a comedic-harem anime but behind the scene, there always someone that schemed behind the scene. Shiro then thought of why he didn't improve so much since the first few years but now he knows. His motivation is to have fun he thought of this world as a game, but now this is his life he needed strength to survive. Soon Shiro would know the effect of his new mindset, it would changes the DxD world forever.
Ignited with the new mindset were Akira and Ichie, They both wanted to be useful for Shiro when they saw Autumn and Iris they felt they're powerless in front of such power. They also didn't want to be left behind by Shiro as he's their only family. (After their contract they see Shiro as a part of their family but even more so when he helped them avenging their clans.)
Shiro then looked at Ichie and Akira's eyes. He then said, "Let's train again but this time it would be for real. We wouldn't stop until we had enough strength to roam freely in this world." Shiro then holds his hand out, "I just wanted to tell you that we might die in the process of obtaining strength, would you guys like to join me?"
"Of course we would Nya~," Ichie said, "We inseparable by now, and your our benefactor, savior, and family. We would be with you through hell." Akira nodded in agreement at Ichie words, "The same here, you're always there for us and we would always be there with you." They both then hold out their hands and hold Shiro's hands.
Shiro expression softened and he smiled at them gently, with wind breezing through them and sun shines at them, as if the sky itself was supporting their decision. He then looked towards a tall mountain in the middle of the island, he never went back to that mountain ever since the first time he'd been there. Thinking back it sent shivers through his spine.
-Flashback-
In his first year, Shiro had traveled around the island and the only one left was this tall mountain, he saves this place for last because he couldn't even see the peak of the mountain. He walked towards the mountain at first it was easy but the more he climbed the more pressure he felt.
He felt that the gravity was stronger and the cold breezes were freezing even him the supposed beast of winter which should be immune to any cold. He felt like he could fall any moments now, but he persevered and got to about 5km marks but then his body just gave up on him and he fell back to the bottom of the mountains.
Below he had marked the last place he could climb to 4km easily but over that the pressure suddenly multiply and he thinks that he isn't at the quarter mark on the mountain but he did found something interesting, It seems the mountain itself create a time distortion that makes it dilation to 1:1000 as he could see that the sun was where it was when he started climbing. He then decided to call the mountain 'The limitless Summit' as he thinks that it's limitless and its peak is nowhere to be seen at least for the current him.
(Note that this is when Shiro strength is High class)
-Flashbacks End-
"I guess it's time for us to climb the 'Limitless Peak'" Shiro said pointing at the mountain. Ichie and Akira just gulped when they hear it as their clan used to tell them stories about the mountain that everyone that climbed it never returned, and there's a being guarding the mountain, Which would reward someone who reaches the peak of the mountain.
But with their new goal set, sooner or later they would climb that mountain anyway so why delay the inevitable. They just nodded, so Shiro led them back to the hut to prepare for the climbs which would take a long time to achieve. He felt that he needed to get stronger fast and that something bad would happen if he isn't strong enough.
-Line Break-
Meanwhile, Iris was trying to take over the spring island since she had full control over Autumn which makes her owns the island. When she failed to went inside the Winter island she successfully left some long-lasting impression with Spring. She met with her and talks to her about stuff that Summer was doing even though at first Spring doesn't believe her with some subtle mind controls she managed to convince her to do so.
Spring herself was an embodiment of life so she believes that people always act for the greater good at first she felt that something was wrong with Iris as her soul isn't pure but Iris told her that summer had tainted her thus making her soul corrupted. Iris herself smirked that Spring was so naive that she believes everything a stranger says. She then spent several months with her and then mysteriously disappeared from the island and she masked her magic signature to prevent Spring from finding her. Making spring worried about her and she searched for Iris to no avail and decided that the other beast was doing so making her distrust others.
This time Iris would go back to the island and make a fabricated story and tried to convince spring to help her.
=Notes=
Well if you wonder why Shiro didn't use gluttony after he beats Autumn because there were too many things that could go wrong, like Iris and the fact that Autumn also didn't take him seriously and he still had his beast form. Shiro also needed to make sure that Ichie and Akira are safe since he didn't want his companions dying on him.