The French marshal who pursued the Prussians instead of fighting at Waterloo was Emmanuel de Grouchy, usually identified simply as Grouchy.
After Napoleon defeated the Prussians at Ligny on 16 June 1815, he ordered Grouchy to lead roughly 33,000 men in pursuit of their retreating army. Napoleon took the main force west to confront Wellington, hoping to defeat the Allied armies separately before they could combine.
Grouchy followed the Prussians toward Wavre, but delays, uncertainty about their route, and disputed interpretations of Napoleon’s orders kept him from reaching Waterloo. On 18 June, while the main battle raged, Grouchy engaged the Prussian rearguard at Wavre. The French won there tactically, but most of the Prussian army had already marched to support Wellington.
The common mix-up is with Marshal Ney, who commanded French forces at Quatre Bras and was present at Waterloo. Soult was Napoleon’s chief of staff, while Davout remained in France until after Napoleon’s defeat.