Sounds similar to YUI, Nano, buzzG, let me try to recall others. Look for: Ikimonogakari's Album - Balladon (Planetarium; Kaerinakunattayo; Sakura; Aruiteiyukou; Futari) Nano's Album - N (Kaleidoscope; Be Free; Horizon; Palette; Nevereverland) Nano's Album - Rock On (Small & Significant; One Life) will recall others if i have time. mileage may vary lel. for one, Nano's music is more aggressive. Ikimono is more lively or soothing? idk.
Nano is the closest to anything I can recall, maybe also Yasuda Rei? Idk if these are what you're looking for since they're more upbeat, just leaving them here if u want something with more strong vocals (I think? Not sure if this is considered strong vocals )