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Gen knew that Senku would never admit it out loud, but he cared about others. Who looked closer at the scientist saw it and Gen always liked to analyze his friends. While he was a genius when it came to science, Senku was clearly lacking in the social department. However there was one person who brought Senku's caring side out the most and Gen was sure that Senku didn't even realize that it was happening. Of course it had to be someone special, someone who was not only a skilled huntress but also the strongest woman the Stone World had to offer.
After a while Gen had learned that Kohaku was the first person from this world who Senku had met and that she was the reason why the village began to trust him and his science. While that was sure to forge a bond between them there was more to it.
The first time Senku had called her lioness in front of him, Gen had thought that it was in a sarcastic way, but the more times he heard it the more he thought that it was a compliment. Senku was far too direct to be anywhere near smooth enough to say something to flatter anyone. When he said something he meant it. He must have analyzed Kohaku as a person and the queen of the animals with her unique distinctive features had seemed the most fitting. Gen could see it clearly. Kohaku was fierce when it came to protecting those she loved, she was the strongest in her village, brave and very caring to her loved ones. Senku, ever the scientist, must have seen the similarities and just thought that nickname was perfect for her. That admiration was included in calling Kohaku a lioness and therefor a compliment was surely something Senku wasn't even aware of. However Gen suspected that Kohaku noticed it because Senku didn't get the same treatment as Chrome. Then again Kohaku was more in touch with her feelings than Senku would ever be.
Considering his newest observations Gen wanted to test a theory and while he hadn't thought that it would be in such a dangerous situation, he should have know better. It couldn't be called a real kiss, but it was enough to make up a believable story and not only that. For Gen it was another piece of the puzzle that was the connection between Senku and Kohaku. Everyone might have thought that their kiss was a sign of romance between them, but for Gen it was more meaningful. Senku's reaction might not show deep romantic love, but it showed that he didn't mind Kohaku's closeness. For someone like Senku that was a huge step. While the scientist showed his appreciation and friendship with high fives or maybe on the best of days a clap on the back, this was different.
The closeness between them stayed after that or maybe it had been there before and Gen was only now paying attention to it. There were no big gestures or moments that made it too obvious. Considering that it was Senku it was more subtle. The way him and Kohaku stood closer to each other than he would with anyone else or when he casual removed her hairband when they got ready for infiltrating the enemy.
Their time on Treasure Island brought Gen's focus on another important aspect of Senku and Kohaku's relationship. Something that mattered a lot to Senku was trust. There was no doubt that Senku trusted them with helping in the steps to bring back the old world step by step, but he trusted Kohaku in another way. It showed in simple things, but the most obvious clue was that he had trusted her not only with extracting the last message of his father but also with the spy mission. This trust was mutual. Something that was confirmed in its depth to Gen when Amaryllis told him that Kohaku had purposefully let herself be petrified. A risk she took because she had full trust in Senku and his ability to save Ginro and her.
When Amaryllis told them about Kohaku and Ginro's petrification Gen watched Senku's emotions get the best of him. It was only for a second and only when everyone else was focused on the other information Amaryllis shared, but it was there. Now Gen was sure that Senku valued Ginro in his own way, but the reaction was clearly directed at the news about Kohaku's sacrifice. It hit him harder than he wanted to admit and he was clearly worried. Just that one second was enough to tell Gen how important she was to Senku.
However what really showed how much affection the scientist held for her was the embrace they shared. The kiss had been staged and while Senku hadn't minded it, it was nothing he stood behind. This embrace was different. Kohaku had been the one to initiate it, but Senku had turned to her and knowing it was coming welcomed it. When she embraced him he even leaned into it and Gen saw the soft look in Senku's eyes. So far Gen had thought that Senku wasn't aware of his growing feelings for Kohaku, but now he wasn't so sure. At least he must be aware of her importance in his life. If Senku was able to realize what that importance could turn into was unknown to Gen.
When a new morning rose on Treasure Island Gen witnessed what clearly shouldn't have been for his eyes. Normally Senku was rational, but this time his emotions broke free and of course it had to be with the person who he trusted the most. Kohaku gave him a strength that was so much more than her physical strength. He could rely on her, something Gen was sure he wasn't used to in this time. Sure, he trusted all of them with tasks, but he didn't show them his vulnerable side. With Kohaku he must feel safe in more than a physical sense. Emotions were something that Senku hid most of the time, but not with her in this moment. He valued her and let her closer to his heart than all of them. However that he was able to realize it and even act on it surprised even Gen.
When Kohaku jumped out of the tree and stood next to Senku he moved closer to her so their shoulders touched. Softly he brushed the back of his hand against hers and their fingers intertwined. Together they watched the sunrise, unaware of Gen who observed them. This was a special moment for them. Senku had initiated the contact for the first time and even if it was far from the romantic kiss a couple shared while watching the sunrise, this was even more meaningful when it came to those two.