Sayotsugu

Sayotsugu is the femslash ship between Sayo Hikawa and Tsugumi Hazawa from the BanG Dream! fandom.

Canon
Although the two had never crossed paths before and did not have a particular reason to interact, both Sayo and Tsugumi can, without a doubt, currently be considered good friends, successfully complementing each other's personality traits. So, the girls are actually the closest friends to each other outside of their groups.

Cooking Class Craziness!
Through a poster outside the shop, Sayo was informed that Hazawa Coffee would be hosting a cookie-baking class, during which Tsugumi got assigned the role of a helper. Due to the image she had of her, Tsugumi was surprised to see that Sayo had showed cooking class that day. Although nervous at first, Tsugumi endured helping Sayo, who appeared to be struggling, to follow the instructions and successfully bake the cookies. The two unexpectedly discovered their common characteristics through the cooking class.

Hazawa Tsugumi - Afternoon Coffee (Memorial Episode: A Day in Hazawa Coffee)
Sayo comes to Tsugumi's cafe to return a book she lent her and have a cup of coffee. During their exchange, it is apparent that both of them have become very good friends who even learned to understand each other's tastes and thinking in literature. So much so that the girls coincidentally recommends each other the same book, thinking that the other would be interested in reading it.

In one of the dialogues, Sayo also mentions that she thinks Tsugumi is the kind of person who can detect beauty even in people the rest of the world ignores. This is quite a significant detail, as in the past, Sayo often felt an inferiority complex due to Hina's talents and even became an obsessive perfectionist because of it.

Additional Content
In 2019, Tsugumi was one of the few characters who received a separate congratulatory animation in the twins' birthday lounge. This is noteworthy, as lounge-room birthday interactions only occur between characters who are fairly close in the lore, or ones who have had significant events together within the past year.

According to one of the limited New Year's Eve interactions at the end of 2020, there was an exchange where Tsugumi invited Sayo to attend her New Year's Eve party, also promising her special coffee and cookies.

Fanon
Despite not having much canon content outside of the "Cooking Class Craziness!" event, Sayotsugu is still one of the most popular ships among fans. This may be due to the fact that both characters come from 5-member bands where the other 4 are often paired up (those being MocaRan, TomoHima, YukiLisa, AkoRinko), so naturally, fans became curious about their interactions as the "odd-ones-out" of the groups. Fans were surprised by their unexpectedly warm and harmonious dynamic that was first introduced in the "Cooking Class Craziness!" event— this is also a big contributing factor to Sayotsugu's popularity, given that Sayo is not one to open up easily.

While this ship was originally considered something of a running gag in fandom due to little character interaction, it has subsequently received some attention from Bushiroad both as a desire to tease popular pairing and some of their attempts to divert fans' attention away from romantically shipping SayoHina due to its incestuous nature. For instance, this ship was teased on a live show episode by Saechi (Tae's seiyuu) when Sayo's seiyuu, KudoHaru, chose Tsugumi as the one she would like to marry amongst all other Garupa characters. This ship has consistently received some minor in-game attention since 2019, either emphasizing the characters' friendly casual interactions or their positive mention of each other in dialogue. So far, their biggest interaction outside of the aforementioned event is the release of a limited card as part of the Kirameki Festival gacha in October 2021.

Tsugumi also began to get noticeable interactions with Sayo's sister, Hina, or even both twins together. Consequently, certain part of the fanbase has began to ship all three as a poly ship or joke about a sibling love-triangle. This triad received attention from Bushiroad as well. As a result, it is mentioned that Tsumugi "has a special effect on Hikawa twins" in one of the events.

As of 2022, Sayotsugu is the most written BanG Dream! ship on AO3. The ship is often rivaled by other popular Sayo ships, such as SayoLisa and SayoHina. SayoTsugu fans' rivalry with the latter is especially infamous, as both ships are the most popular ships for Sayo and two of the most popular ships in fandom in general. Thus, SayoHina fans often accuse the latter of having a toxic fanbase and overestimating the potential of their ship, while SayoTsugu fans can be found calling out Sayohina shippers for promoting incest and sexualizing the twins' relationship.

Fandom
FAN FICTION

PIXIV
 * さよつぐ posts on Pixiv

Trivia

 * The pair is often compared to mint chocolate because of their hair colors.
 * Sayo is 161cm tall while Tsugumi is 156cm.
 * Upon their first notable interaction, Sayo addressed Tsugumi by "Hazawa-san", but a more recent area conversation shows her addressing Tsugumi by her first name instead (though, still keeping the -san honorific)
 * Sayotsugu's alternate ship name "342g" is a play on words with their names (3 - "Sa", 4 - "Yo", 2 - "Tsu", g - "Gu").
 * BanG Dream's first Kirameki Festival Gacha features an animation of Sayo and Tsugumi in one card.
 * SayoTsugu is the only cross-band pairing where girls don't have big ships or canonical best friends mates within their own groups.
 * Tsugu is the second character after Hina who usually appears alongside Sayo when she is shown standing and/or talking to someone in the background in various additional content.