The existence of the neutrino flavour oscillation phenomenon carries a potential CP-violation phase, 𝛿CP, that might be the answer to the matter-antimatter asymmetry question in the Universe. With a new far detector and upgraded components of the successful Tokai-to- Kamioka (T2K) experiment, the long-baseline aspect of the Hyper-Kamiokande (Hyper-K) experiment will utilize an upgraded neutrino beam from the Japan Proton Accelerator Research Complex (J-PARC). The measurement of 𝛿CP from 𝜈𝜇 and ¯ 𝜈𝜇 disappearance modes will be dominated by systematic uncertainties. To reduce these uncertainties to discovery-level precision, an Intermediate Water Cherenkov Detector (IWCD) is introduced at a distance of 1 km from the beam source to intercept the neutrino beam at a span of off-axis angles and energies. This detector will feature high granularity, directionality, and time response through the use of a multi-photomultiplier tube (mPMT) photo-detection system. In this thesis I discuss the research and development (R&D) of scintillator-based detectors for the mPMT system on two main fronts: an internal hit detector for the mPMT system and a time-of-flight detector for particle monitoring and identification in tests of an IWCD prototype, the Water Cherenkov Test Experiment. Both detectors aim to contribute to the reduction of systematic uncertainties in Hyper-K’s attempt to discover 𝛿CP in the lepton sector of the Standard Model.. Keywords: Neutrino oscillation, Hyper-Kamiokande, Intermediate Water Cherenkov Detector, multiPMT, leptonic CP-violation phase, scintillator, time-of-flight detector.