SCHEDULE 13G/A 0002025342 XXXXXXXX LIVE 4 Common Stock, par value $0.01 per share 09/30/2025 0001990354 Waystar Holding Corp. 946784105 1550 Digital Drive, #300 Lehi UT 84043 Rule 13d-1(d) BCPE Derby Investor, LP b DE 0 1913839 0 1913839 1913839 N 1.0 PN BCPE Derby (DE) SPV, LP b DE 0 11329700 0 11329700 11329700 N 5.9 PN Waystar Holding Corp. 1550 Digital Drive, #300, Lehi, Utah 84043 This Schedule 13G is being filed jointly by BCPE Derby Investor, LP, a Delaware limited partnership ("BCPE Derby"), and BCPE Derby (DE) SPV, LP, a Delaware limtied partnership ("BCPE Derby SPV" and, together with BCPE Derby, the "Reporting Persons"). Bain Capital Investors, LLC ("BCI"), a Delaware limited liability company, is the manager of Bain Capital Partners XI, LP, a Cayman Islands exempted limited partnership ("Partners XI"), which is the general partner of Bain Capital Fund XI, LP, a Cayman Islands exempted limited partnership ("Fund XI"). Fund XI is the (i) sole member of BCPE Derby GP, LLC, a Delaware limited liability company ("BCPE Derby GP"), which is the general partner of BCPE Derby, and (ii) sole member of BCPE Derby (DE) SPV, LLC, a Delaware limtied liability company ("BCPE Derby SPV GP" and, together with the Reporting Persons, BCI, Partners XI, Fund XI and BCPE Derby GP, the "Bain Capital Entities"), which is the general partner of BCPE Derby SPV. As a result, BCI may be deemed to share voting and dispositive power with respect to the securities held by the Reportin Persons. Voting and investment decisions with respect to securities held by the Reporting Persons are made by the partners of BCI. The Reporting Persons have entered into a Joint Filing Agreement, dated November 14, 2025, pursuant to which the Reporting Persons have agreed to file this Schedule 13G jointly in accordance with the provisions of Rule 13d-1(k)(1) promulgated under the Act. The principal business address for each of the Bain Capital Entities is 200 Clarendon Street, Boston, Massachusetts 02116. The Reporting Persons, BCI, BCPE Derby GP and BCPE Derby SPV GP are each organized under the laws of the State of Delaware. Partners XI and Fund XI are each organized under the laws of the Cayman Islands. Y As of the close of business on September 30, 2025, BCPE Derby held 1,913,839 shares of common stock, representing approximately 1.0% of the outstanding common stock, and BCPE Derby SPV held 11,329,700 shares of common stock, representing approximately 5.9% of the outstanding common stock. As a result of the foregoing and the relationships described in Item 2(a) of this Schedule 13G, the Reporting Persons may be deemed to collectively beneficially own an aggregate of 13,243,539 shares of common stock, representing approximately 6.9% of the outstanding common stock. The percentage of the outstanding shares of common stock held by the Reporting Persons is based on 191,316,583 shares of common stock issued and outstanding, as reported by the Issuer in its Quarterly Report on Form 10-Q filed with the Securities and Exchange Commission on October 29, 2025. The Reporting Persons and certain other stockholders are parties to a Stockholders Agreement (the "Agreement") with the Issuer, which was filed on a Form 8-K by the Issuer on June 12, 2024. The Agreement provides the Reporting Persons with the right to nominate one director to the Issuer's board of directors for so long as the Reporting Persons hold 5% or more of the then-outstanding common stock. The Agreement further requires that the Reporting Persons and other parties to the Agreement that hold similar nomination rights vote in favor of those nominated pursuant to the Agreement and, as such, may be deemed to constitute a group for purposes of Section 13(d) under the Securities Exchange Act of 1934, as amended. Notwithstanding the foregoing, the Reporting Persons expressly disclaim membership in such group and the filing of this Statement shall not be deemed an admission of such group membership or beneficial ownership of the securities held by the other parties to the Agreement. See Item 4(a) hereof. 0 See Item 4(a) hereof. 0 See Item 4(a) hereof. Y Y Y N See Item 4(a) hereof. Y Y BCPE Derby Investor, LP /s/ David Humphrey 11/14/2025 BCPE Derby (DE) SPV, LP /s/ David Humphrey 11/14/2025