THIS IS A SPOILER WARNING
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FINAL WARNING FOR SPOILERS
The person who killed Regina is the orphanage teacher Connie. The reason is because she was madly in love with Leo's sperm donor, the red haired dude from the Olor (I forgot if it'sthis name used in the manhwa, it's the swan family guy) family. Connie's real name is Saura, she helped Regina and Olor escape from the north years ago, she was the one drving the carriage and staged the fall into the river. Saura thought that swan dude would finally reciprocate her love, but no, he didn't , so she kidnapped Regina whilst she was pregnant, which no one knew about, not even the sperm donor. After Leo was born, Saura killed Regina, but Regina seemingly getting some smart in her (don't mean to be disrespectful to the dead, but this is Regina we're talking about ) swallowed a swan necklace given to her by sperm donor, so that if her body was found, they would know where to start. When Leo finds out about it she gets so heartbroken that everyone around her noticed, because in Phileo's words, she no longer squeezed his thigh muscles