I THINK SO TOOO. When I saw his first (or second?) appearance at the party on the balcony with Ilya (mc) His dark hair shines a bit reddish at the tip because of the sunset. A few chapters after that, in his flashbacks, Mc's ex found her lying on the field of grass. My guess is Karhan was the one who pulled her out of the water, and her ex was just claiming that he was the one who saved her.
Her ex found her and called the servants ig and ppl around her said he was the one who saved her. I forgot the details. Anyway, it was alr explained that he didn't have that much interest in her except her wealth. There was this one whole section/chapter that was dedicated solely for his background motives, thoughts, and hints.
Yes, after i read the latest chapter my brain automatically came with the conclusion that it was Karhan that saved her. Knowing his personality, he probably thought that if he were to ask for help for Ilya to be looked after by a doctor after saving her from the drowning incident that people would've jumped to conclusions that it was probably his fault and maybe wanted to avoid the consequences of that. Thats why he left her in a safe grass field to be seen by someone who might help her later on?. But as you said that Rehart trash found her and decided to use her and take advantage of her just for her wealth.
The one who saved her from drowning that day was none other than our sweet, shy Karhan. Due to the fact of her almost fainting she didn't see the man's face clearly and because the sunset was hitting his hair she mistook his beautiful dark hair with red.
Why, my guts it's telling me definitely was Karhan.