The supporting cast of Attack on Titan is famously just as robust and fully developed as its main characters, and they often drive the plot to the same degree as its main cast. Because of this, the diverse and well-rounded supporting characters allow the series to cover not just more perspectives but also more genres. A key example of this is the one and only Sasha Braus.
Source:https://www.cbr.com/aot-why-sasha-is-best-character/