Amelia Hawkbridge is a named character. She played the supporting role in the novel and was someone who shared a close bond with Ethan...but just as a friend.
Amelia possesses some rare skills and very blunt behaviour. She performs well as a scout and someone who would be beneficial for Ethan in his journey.
At some point, Amelia realises her feelings for Ethan, but the person who always remained direct with her feelings and never hesitated from sharing her thoughts, never found the courage to tell him about her feelings.
A losing heroine, to be precise.
And currently that losing heroine was sharing the carriage with Richard. Canonically, Richard's engagement was annulled while he was in the academy and after the annulment, Richard died not so long after. So it is understandable why this sudden plot shift occurred.
Now that Richard was single, who could be a better fiancé for Amelia other than the heir of Scarlette's house?