Эп. 13: Тома reconnects with childhood friend Kotomi after her tragic loss and isolation

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Tomoya realizes that he met Kotomi as a kid, though only she had remembered. Back then, Tomoya had often visited Kotomi, and was her only friend. Around this time on Kotomi's birthday, her parents left for business reasons, and later that day she finds out they were killed in a plane crash. Kotomi ultimately ended up burning her father's important work before anyone could see it. While Kotomi shuts herself in her house, Tomoya comes over and starts cleaning up the garden which is overgrown with grass and weeds. Nagisa, Kyou and Ryou eventually help out too. (Source: Clannad Wikia)