It all starts out with Croissant Cookie:
- Croissant experiments by entering a time rift without goggles on. This scars her eyes and causes her to crash her Timecraft, rendering it useless and in need of repairs. Instead of repairing the craft, Croissant decides to upgrade her Timeweaver Scissors, and they become the Sonic Embroider, a more powerful version that does not require a Timecraft to travel through rifts. Normally, the Craft AND the Scissors are required to be able to actually open AND enter a rift in time.
- Croissant calls the TBD to request help in bringing the totaled Timecraft back to her garage. At this point, her goggles are key in making things seem like they were okay; she didn't want to alarm anyone to what she was doing.
- Instead of repairing the craft, Croissant continues experimenting with the Sonic Embroider. This leads to the second stage of the transformation.
After the crash:
- Croissant still attends university with her friends Hero Cookie and Sandwich Cookie. Sandwich Cookie drops out of university almost a month before Croissant's accident happens to pursue her dream of working in the sandwich shop, so only Hero notices Croissant's constant use of her goggles. Despite multiple questions, Croissant denies taking off the goggles, and slowly becomes more distant, worrying Hero.
- The changes to Croissant's skin and hair begin: her skin becomes grayer and her hair straightens and puffs out to match the present Timekeeper's, and her left eye begins to see infinite timelines. It only takes about a month of this phenomenon for Croissant to once again request a blackout eye-patch (similar to a blackout sleeping mask) to curb her left eye's sight. Almost uncannily, the eye-patch she receives matches the shade of her yellow eyes almost perfectly.
- A month passes by, and the effects worsen. Croissant stops going to university, and disappears from the TBD's ranks entirely. Hero notices this change, and plans to approach her with Sandwich's help.
- Hero and Sandwich confront Croissant, who is just returning from some time in the time rift. A conflict is had, but Hero and Sandwich leave eventually, distraught, discouraged, and having lost their friend. Croissant simply returns to the rift, hoping now to speed up the process. After another month, the changes become prominent enough for a wardrobe and name change to become completely possible, and the new Timekeeper Cookie vanishes from the timeline forever.
After abandoning their timeline:
- Timekeeper Cookie continues to experiment with visiting other timelines, messing with them and even ending some entirely (think about the timelines cancelled/older Cookie Run games follow (ex. LINE Cookie Run, Cookie Wars, etc.)). Eventually, Timekeeper begins to look for their counterparts, initially only finding Croissant Cookies who never joined the TBD. Searching takes nearly a year (in relative time rift time) until Timekeeper finds the Croissant Cookie we know in Ovenbreak. Keeping this in mind, Timekeeper begins to plan.
- Going back in time, Timekeeper enters the new timeline and confronts the old Director of the TBD. Of course, this director would be unknown for obvious reasons.
- Timekeeper overthrows this old Director, but keeps their identity a secrets from the TBD itself, save for the fact that there is a new Director, hence why no one has ever seen them before during Operation Timeguard's events.
- The new Croissant joins the TBD while Timekeeper is Director, solidifying their involvement in the upcoming Timeguard event.
- Gingerbrave is soon accused of messing with the timeline, causing the events of Operation Timeguard.
Hope this headcanon sheet seems solid!