KCG Kcg Holdings Class A

CUSIP No. 48244B 100

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CUSIP No. 48244B 100

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2,022,032 Class A Common Shares and 1,743,918 Warrant Shares reported in the table above are held directly by the Schuler Family GST Trust. Stephen G. Schuler may be deemed, for purposes of Rule 13d-3 under the Securities Exchange Act of 1934, as amended, to share voting and dispositive power with respect to these securities. Neither the filing of this Schedule 13D nor any of its contents shall be deemed to constitute an admission by the Reporting Person that he is the beneficial owner of any of such securities for purposes of Section 13(d) of the Securities Exchange Act of 1934, as amended, or for any other purpose.

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CUSIP No. 48244B 100

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2,022,032 Class A Common Shares and 1,743,918 Warrant Shares reported in the table above are held directly by the Schuler Family GST Trust. Mary Jo Schuler may be deemed, for purposes of Rule 13d-3 under the Securities Exchange Act of 1934, as amended, to share voting and dispositive power with respect to these securities. Neither the filing of this Schedule 13D nor any of its contents shall be deemed to constitute an admission by the Reporting Person that she is the beneficial owner of any of such securities for purposes of Section 13(d) of the Securities Exchange Act of 1934, as amended, or for any other purpose.

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Introduction

This Amendment No. 6 (this “Amendment No. 6”) amends Amendment No. 5 dated March 3, 2014 (“Amendment No. 5”), Amendment No. 4 dated February 3, 2014 (“Amendment No. 4”), Amendment No. 3 dated December 31, 2013 (“Amendment No. 3”), Amendment No. 2 dated December 9, 2013 (“Amendment No. 2”) and Amendment No. 1 dated November 1, 2013 (“Amendment No. 1”) to the statement on Schedule 13D dated July 1, 2013 (the “Original Statement” and, together with Amendment No. 1, Amendment No. 2, Amendment No. 3, Amendment No. 4, Amendment No. 5 and this Amendment No. 6, this “Schedule 13D”) relating to the Class A common stock, par value $0.01 per share (the “Class A Common Shares”), of KCG Holdings, Inc., a Delaware corporation (“KCG Holdings”). Except as specifically provided herein, this Amendment No. 6 does not modify any of the information previously reported in the Original Statement. Any capitalized terms used in this Amendment No. 6 and not otherwise defined herein shall have the respective meanings ascribed to such terms in the Original Statement.

This Amendment No. 6 reflects transactions and developments through the date hereof relating to the Reporting Persons’ holdings of Class A Common Shares. In particular, this Amendment No. 6 is being filed to reflect a redemption of certain membership interests in Serenity Investments, LLC, a limited liability company organized under the laws of the state of Alaska (“Serenity”).

There has been no change to the information disclosed in Item 1 of the Original Statement.

There has been no change to the information disclosed in Item 2 of the Original Statement.

There has been no change to the information disclosed in Item 3 of the Original Statement.

There has been no change to the information disclosed in Item 4 of the Original Statement, and the information disclosed in Item 4 of Amendment No. 1 is hereby superseded.

Item 5 of the Schedule 13D is hereby amended and restated in its entirety as follows:

(a) - (b) With respect to Serenity, Stephen G. Schuler and Mary Jo Schuler, this Schedule 13D relates to the beneficial ownership of 9,565,516 Class A Common Shares, which include (i) 6,596,485 Class A Common Shares and (ii) 2,969,031 Warrant Shares. Such 9,565,516 shares represent 7.6% of the outstanding Class A Common Shares (based on a total of 128,131,518 Class A Common Shares outstanding, which consists of (i) an estimated 125,162,487 Class A Common Shares outstanding as of May 8, 2014, according to information filed by KCG Holdings on May 12, 2014, and (ii) 2,969,031 Warrant Shares). All of these Class A Common Shares and Warrant Shares are held by Serenity. Stephen G. Schuler and Mary Jo Schuler separately hold equity interests in Serenity that together represent a controlling equity interest in Serenity. Each of Serenity, Stephen G. Schuler and Mary Jo Schuler may be deemed to share voting and dispositive power with respect to these securities. Each of Stephen G. Schuler and Mary Jo Schuler disclaims beneficial ownership of these securities except to the extent of his or her pecuniary interest therein. Neither the filing of this Schedule 13D nor any of its contents shall be deemed to constitute an admission by Stephen G. Schuler or Mary Jo Schuler that he or she is the beneficial owner of any of such securities referred to herein for purposes of Section 13(d) of the Securities Exchange Act of 1934, as amended (the “Exchange Act”), or for any other purpose.

Solely with respect to Stephen G. Schuler and Mary Jo Schuler, this Schedule 13D also relates to the beneficial ownership of an additional 3,773,171 Class A Common Shares, which include (i) 2,029,253 Class A

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Common Shares and (ii) 1,743,918 Warrant Shares. Together with the 9,565,516 shares described in the preceding paragraph, these shares represent 10.3% of the outstanding Class A Common Shares (based on a total of 129,875,436 Class A Common Shares outstanding, which consists of (i) an estimated 125,162,487 Class A Common Shares outstanding as of May 8, 2014, according to information filed by KCG Holdings on May 12, 2014, and (ii) 4,712,949 Warrant Shares). Of those, 3,765,950 Class A Common Shares and Warrant Shares are held by the Schuler Family GST Trust. Stephen G. Schuler and Mary Jo Schuler may be deemed, for purposes of Rule 13d-3 under the Exchange Act, to share voting and dispositive power with respect to these securities. Stephen G. Schuler and Mary Jo Schuler disclaim beneficial ownership of these securities. Neither the filing of this Schedule 13D nor any of its contents shall be deemed to constitute an admission by Stephen G. Schuler or Mary Jo Schuler that he or she is the beneficial owner of any of such securities for purposes of Section 13(d) of the Exchange Act or for any other purpose.

(c) On July 15, 2014, the redemption of the 12.1% membership of the Schuler Family GST Trust in Serenity was completed and, in connection therewith, an additional 564,766 Class A Common Shares and 1,173,651 Warrant Shares held by Serenity were transferred to the Schuler Family GST Trust. No Reporting Person has effected any other transaction in the Class A Common Shares or Warrant Shares since the filing of Amendment No. 5.

(d) To the knowledge of the Reporting Persons, no other person has the right to receive or the power to direct the receipt of dividends from, or the proceeds from the sale of, the Class A Common Shares or Warrant Shares covered by this Schedule 13D.

(e) Not applicable.

On March 3, 2014, Serenity sold 3,000,000 Class A Common Shares at a price of $11.00 per share to Jefferies LLC in a private transaction. In connection therewith, Serenity agreed not to sell, without the prior consent of Jefferies LLC, any Class A Common Shares during the six month period after the date thereof. No Reporting Person has any other contracts, arrangements, understandings or relationships with respect to any securities of KCG Holdings.

There has been no change to the information disclosed in Item 7 of the Original Statement.

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SIGNATURES

After reasonable inquiry and to the best of my knowledge and belief, I certify that the information set forth in this statement is true, complete and correct.

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