If your curious here’s a TLDR of why she exists, in the next post of her I’ll tell more about her:
Long story short when the Rainboom happened, it brought the main six their cutiemarks. This being something that they learn way later when they chat together. (in this alternative timeline) Twilight gotten a realization, If Rainbow Dash was the one who helped them get their cutiemark all at the same time; that means that she is the one who lead us all to be where they are today. So after some thinking she went to Rainbow and proposed to make her the leader of the friend group, since she had done something so monumental.
This however shocked and scared Rainbow Dash, as much as her ego is to the roof when it comes to showing off and being awesome, being a leader (especially by herself) was challenging to her. Twilight felt lost. She knew something had to be worked on, she had the capabilities of helping with leading the group with confidence, something Twilight lacked at times especially with her anxiety making her explode with emotions. Rainbow wondered if the both of them would make for a great team, both together being the leaders to help the others with adventures they may set on.
This then clicked to Twilight. When she studied in Canterlot, she often read about the tales of Star Swirl the bearded and the many MANY spells he had conjured up during his time. One of them happen to be a fusion spell. Star Swirl didn’t use it that often but kept it in his studies on the off chance they would ever need to use it. With this Twilight proposes he idea of her learning this fusion spell and essentially fusing them together.
Rainbow Dash was even more scared at such an option, she’d loose her ability to try out for the Wonderbolts, or worse, loose her Rainbow mane and ability to fly! Twilight reassured her that the spell is suppose to combine their powers together into one coherent form. Rather they would mesh beautifully into a fusion that didn’t look like a fusion at all but a individual. All the while keeping the abilites that both have into one that can work for either side of the fusion. After sometime of convincing, she finally agreed, Twilight went to the archives to seek for the spell. To great success she was able to find it and went back to her home where Rainbow dash was swinging back and forth from a stool. Then she ignited the spell and becoming the fusion, Rainbow Sparkle.
Aw Cute story and art. Though Twilight turning their group on it’s head because of something Rainbow did as a filly is a bit bizarre but I guess that’s a minor detail.