Sonia G Lotus Soft Definer is a brush.
Lotus Soft Definer is a very small crease brush with a soft, rounded edge that opened and flared out slightly after the initial wash (which was as intended). It features dyed saikoho goat bristles, and it has a hair length of 12mm and total length of 152mm. This is a new and (and at least, at this time) exclusive brush to the set.
From the set, this was the one brush that stood out to be the most. The brand described it as a "cross between a pencil brush and a crease brush," which is exactly how I'd describe it. The length is more like a small crease brush, but the precision and firmness is more like a pencil brush. This is great for getting a more defined crease color placement or ensuring more intensity in the crease. Often, I use a smaller crease brush to initially deposit and lightly blend out crease colors, especially in the deepest part of my crease, and then use a larger, fluffier brush to really diffuse and blend out the edge.
It's slightly shorter, more compact, and firmer with less flexibility/fluffiness compared to the Mini Booster, which is one I use often and have purchased multiples over. Between the two, the Mini Booster is more versatile, but I expect I'll be reaching for the Soft Definer often.