The Lake

The Lake, or more accurately referred to as The Park, is a recurring location in the 5-toubun no Hanayome series. This place is heavily based on real-life Nakanoike Park (中ノ池公園) in,. The lake is located presumably near the Uesugi Family's Residence due to being in a walking distance from it.

Seven Goodbyes Arc
After Itsuki's sudden intrusion, she suggests for a night walk to talk with Fuutarou. At the lake, Itsuki reveals about the quintuplets' poverty prior to their mother's remarriage to their stepfather and her inspiration to become like her mother. Additionally, Fuutarou finally reveals his reason to study. At that moment, Fuutarou senses a mysterious person nearby as there is a rustling noises behind them; so far, its identity was never explicitly stated.

A few days later, Fuutarou meets Rena at the lake who introduces that she is the girl that he met 5 years ago in Kyoto after seeing the charm and comparing the photo to her. He is forced into a boat ride with her since she takes his student handbook as hostage. Their conversation ends after the automated fountain showers onto them forcing Fuutarou to roll back to the dock. Rena leaves the boat first and returned Fuutarou's student handbook to him; however, she took the photo and told him that she cannot see him again. Trying to catch Rena, Fuutarou falls into the lake. Rena says goodbye and disappears.

Yotsuba finds a soaking wet Fuutarou and unnaturally asks about his condition before the track club's captain rushes her to leave. Wet and tired, Fuutarou leaves the lake and heads to Nino's hotel.

Kyoto Trip Arc
The lake is revisited after the Kyoto trip. Fuutarou meets up with Rena - their means of contacting was never shown - and gave her his birthday present which is an album of the sisters' trip in Kyoto, secretly taken by Fuutarou, Maeda, and possibly Takeda.

Despite knowing that it is one of the five but not knowing who, Fuutarou conveys his gratitude toward her and her presence 5 years ago before leaving the place. A while after, Rena expresses her concern to reveal the identity of the girl from 5 years ago to someone.

Trivia

 * Nakanoike (中ノ池) literally means "within the lake." Another interpretation of this word is "lake Nakano." The surname Nakano (中野) translates into "within the fields", despite the two share similar pronunciation. This could be a deliberate pun from the author when coming up with the Nakano surname.
 * In the manga, there is a boat port at the lake. This place does not exist in the real-life Nakanoike Park. In reality, boats are prohibited in the area. Therefore, this could be a rendition of the location from the author.