There is nothing to suggest that all of Wilhelm's nine illegitimate children were born to just one woman, in fact that seems highly unlikely, however rather than creating nine partners for Wilhelm all named "N.N." i have grouped all the illegitimate children under one mother until further evidence appears.
Comte Guillaume VI had [nine] illegitimate children by unknown mistresses:
2. LODEWIJK (-[20 May/13 Jun] 1433). Heer van Vlissingen, seigneur d'Escaudoeuvres near Cambrai. m (shortly before 2 Sep 1416) ELEONORE de Floyon, daughter of --- (-after 13 Jun 1433). According to Kerrebrouck (who cites no source), he married an illegitimate daughter of Carlos III King of Navarre.
3. JEAN bâtard de Hainaut (-[6 Nov 1432/20 Aug 1436]). m MARIE Dame de Warigny, daughter of --- (-after 20 Aug 1436).
4. WILLEM (-Bruges, Kloster Genadendal 23 Apr 1455). In Kloster Genadendal in Bruges 1407 'clericus redditus'. m (before 12 Aug 1416) BEATRIX van Hodenpijl, daughter of JAN van Hodenpijl & his wife ---.
5. BEATRIX (-1455). m firstly (18 Jun 1410) FILIP van Veen gt van Dorp, son of --- (-[18 Jun 1410/17 Sep 1411], bur Delft oude Kerk). Rentmeester of Nordholland 1400-02. Treasurer of Holland. m secondly ([Schoonhoven 19 Oct 1411]) JAN van Vliet Heer van Hoenkoop, son of --- (-The Hague, hanged, drawn and quartered for murder 3 Aug 1424). Hofmeister of Willem VI Count of Holland. He attempted to murder Jan Count of Holland, Hainaut and Seeland, Bishop-elect of Liège by poisoning the pages of his bible.
6. EBERHARD Heer van Hoogwoude (-20 Mar 1458). m firstly (shortly before 24 Jul 1430) JUTTA van Kijfhoeck, daughter of FLORIS van Kijfhoeck & his wife --- van Rossum (-before 23 May 1449). m secondly (before 12 Mar 1467) as her second husband, ALEID van Swieten, widow of CLAAS van Diepenburch, daughter of BOUDEWIJN van Swieten & his wife Lutgard van Nijenrode (-12 Mar 1467, bur The Hague). Eberhard & his first wife had children.
7. BAUDOUIN . Seigneur de Plainville. m MARGUERITE de Rouvroy dit de Saint-Simon, daughter of ---.
8. CHRISTINA . 1436.
9. [HENDRIK bastaard van Holland (-after 7 Jul 1427).]
10. [ARENT . m ALEID Uterlier van Dorp, daughter of SIMON Uterlier van Dorp & his wife Clementia van de Boekhorst.]