This is all the information I could find on this topic:

  The origins of the kite shield remain unknown [1]. However, the first known illustration of a kite shield appeared in the Gospels of Otto III, which was made between 983 - 991, as part of the armor of a Norman guard [3].

  We can also find depictions of this shield in art and literature through the eleventh and twelfth century [2]. In the case of the Bizantine Empire, there are depictions of its adoption since the eleventh century [2].

 

1. Grotowski, P., & Brzezinski, R. (2010). Arms and Armour of the Warrior Saints. BRILL. pp. 233.
2. Ibid. pp. 232.
3. R. Ewart Oakeshott (1960). The Archaeology of Weapons: Arms and Armor from Prehistory to the Age of Chivalry. COURIER CORPORATION. pp. 176

If we're talking about the Western Roman Empire, it never was introduced. The classic scutum was not ovular, but more of a concave rectangle. By the 4th century, these had largely been replaced by flat, ovular shields.

The kite shield was a high medieval development, as /u/ikko001 points out, with more than 500 years between its appearance and the collapse of the WRE.