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The properties of a remnant black hole can be probed by analyzing the gravitational waves emitted during its ringdown phase. This signal provides a direct test of general relativity in the strong-field regime. In this study, we apply both a time-domain <inline-formula><mml:math><mml:mrow><mml:mi>F</mml:mi><mml:mtext>-statistic</mml:mtext></mml:mrow></mml:math></inline-formula> framework and full Bayesian time-domain sampling to the ringdown of GW231028_153006. We report decisive evidence for multimode content, specifically identifying both the first overtone (<inline-formula><mml:math><mml:mrow><mml:mi>ℓ</mml:mi><mml:mo>|</mml:mo><mml:mi>m</mml:mi><mml:mo>|</mml:mo><mml:mi>n</mml:mi><mml:mo>=</mml:mo><mml:mn>221</mml:mn></mml:mrow></mml:math></inline-formula>) and the higher order (<inline-formula><mml:math><mml:mrow><mml:mi>ℓ</mml:mi><mml:mo>|</mml:mo><mml:mi>m</mml:mi><mml:mo>|</mml:mo><mml:mi>n</mml:mi><mml:mo>=</mml:mo><mml:mn>210</mml:mn></mml:mrow></mml:math></inline-formula>). The detection of the 221 mode is statistically significant, achieving a Bayes factor of <inline-formula><mml:math><mml:mrow><mml:mo>∼</mml:mo><mml:mn>189.2</mml:mn></mml:mrow></mml:math></inline-formula> and an amplitude exclusion from zero at <inline-formula><mml:math><mml:mrow><mml:mo>&gt;</mml:mo><mml:mn>7</mml:mn><mml:mi>σ</mml:mi></mml:mrow></mml:math></inline-formula> credibility for an analysis beginning at <inline-formula><mml:math><mml:mrow><mml:mn>10</mml:mn><mml:mi>M</mml:mi></mml:mrow></mml:math></inline-formula> postpeak. These findings are rigorously validated against the numerical relativity injection SXS:BBH:1282, which confirms that such multimode features are physically expected for a remnant with the inferred high spin and mass ratio. The inclusion of the overtone mode allows for precise constraints on the remnant's properties, yielding a redshifted final mass of <inline-formula><mml:math><mml:mrow><mml:mn>246</mml:mn><mml:mo>.</mml:mo><mml:msubsup><mml:mn>2</mml:mn><mml:mrow><mml:mo>−</mml:mo><mml:mn>22.4</mml:mn></mml:mrow><mml:mrow><mml:mo>+</mml:mo><mml:mn>22.3</mml:mn></mml:mrow></mml:msubsup><mml:msub><mml:mi>M</mml:mi><mml:mo>⊙</mml:mo></mml:msub></mml:mrow></mml:math></inline-formula> and a final spin of <inline-formula><mml:math><mml:mrow><mml:mn>0</mml:mn><mml:mo>.</mml:mo><mml:msubsup><mml:mn>81</mml:mn><mml:mrow><mml:mo>−</mml:mo><mml:mn>0.10</mml:mn></mml:mrow><mml:mrow><mml:mo>+</mml:mo><mml:mn>0.07</mml:mn></mml:mrow></mml:msubsup></mml:mrow></mml:math></inline-formula> (at <inline-formula><mml:math><mml:mrow><mml:mn>90</mml:mn><mml:mo>%</mml:mo></mml:mrow></mml:math></inline-formula> credibility), consistent with full inspiral-merger-ringdown predictions. A test of the no-hair theorem, enabled by this robust multimode detection, shows consistency with general relativity.