This work reports a gallium indium alloy (EGaIn)-doped SiBOC ceramic that possesses a unique liquid-metal-in-ceramic feature. The low-viscosity liquid nature of gallium-based liquid metals (Ga-LMs) and the reactive core-shell structure provide possibilities for phase engineering inside polymer-derived ceramics. As a demonstration, EGaIn nanoparticles (NPs) are directly mixed with a UV-curable ceramic precursor (UV-PBS) to obtain a resin suitable for digital light processing 3D-printing. After pyrolysis at 800–1200 °C, SiBOC ceramics with uniformly distributed EGaIn NP domains (Si(GaIn)BOC) are obtained. EGaIn plays a key role in promoting carbonization and preventing crack formation during the polymer-to-ceramic process, resulting in an increase in both ceramic yield and mechanical strength. EGaIn NPs are also found to have a core-shell structure (EGaIn@(GaxIn1-x)2O3@SiBOC) inside the SiBOC matrix, which significantly enhances the dielectric properties and improves the interfacial polarization. As a result, an excellent electromagnetic wave absorption performance is achieved across the C, X, and Ku bands, respectively. Through rational design, a novel metastructure design based on the Schwarz P minimal surface is proposed, which exhibits an ultrawide effective absorption band extending up to 11.36 GHz (within C-Ku bands).